Chemical   Monographs 


SB    272    3^0      • 

L^BBBHHHIB 

CHEMISTRY  OF  DYEING 


JOHN    KERFOOT    WOOD 


,  Van  Nostrand  Company 


MAR  27  1922 


CHEMICAL     MONOGRAPHS 

EDITED  BY  A.  C.  GUMMING,  D.Sc. 


No.  II 

The  Chemistry   of  Dyeing 


CHEMICAL    MONOGRAPHS 

EDITED  BY  A.  C.  GUMMING,  D.Sc. 

THE  progress  of  Chemistry  is  so  rapid  that  it  is 
becoming  a  matter  of  ever-increasing  difficulty  to 
keep  abreast  of  the  modern  developments  of  the 
science.  The  volume  of  periodical  literature  is  so 
enormous  that  few  can  hope  to  read,  far  less 
assimilate,  all  that  is  published.  The  preparation 
of  summaries  has  therefore  become  a  necessity,  and 
has  led  to  the  publication  of  various  well-known 
journals  devoted  to  the  abstraction  of  original  papers. 
For  obvious  reasons,  however,  these  do  not  fully 
supply  the  wants  of  advanced  students  and  research 
workers,  and  it  is  now  generally  recognised  that 
monographs  on  special  subjects  are  also  needed. 

This  series  of  monographs  is  intended  primarily  for 
Advanced  and  Honours  students.  As  each  mono- 
graph is  written  by  an  author  with  special  knowledge 
of  the  subject,  and  copious  references  are  given, 
it  is  hoped  that  the  series  will  prove  useful  also  to 
those  engaged  in  research. 

The  following  volumes  are  ready  or  in  active 
preparation  :— 

THE  ORGANOMETALLIC  COMPOUNDS  OF  ZINC  AND  MAGNESIUM. 
By  HENRY  WREN,  M.A.,  D.Sc.,  PH.D.,  Head  of  the 
Department  of  Pure  and  Applied  Chemistry  at  the  Muni- 
cipal Technical  Institute,  Belfast.  Ready. 

THE  CHEMISTRY  OF  DYEING.  By  JOHN  KERFOOT  WOOD,  D.Sc., 
Lecturer  on  Chemistry,  University  College,  Dundee. 

Ready. 

THE  CHEMISTRY  OF  RUBBER.  By  B.  D.  PORRITT,  F.I.C.,  B.Sc., 
Chief  Chemist  to  the  North  British  Rubber  Company. 

In  the  Press. 

THE  FIXATION  OF  ATMOSPHERIC  NITROGEN.  By  JOSEPH  KNOX, 
D.Sc.,  Lecturer  on  Inorganic  Chemistry,  University  of 
Aberdeen.  In  Active  Preparation. 

Other  Volumes  to  follow. 


THE  CHEMISTRY 
OF    DYEING 


BY 

JOHN   K.  WOOD,  D.Sc. 

Lecturer  in  Chemistry,   University  College,  Dundee 


NEW  YORK 
D.  VAN  NOSTRAND  COMPANY 

EIGHT   WARREN   STREET 
1913 


Kuf :-. ' 


W* 


PREFACE 

IN  writing  this  little  book  my  principal  aim  has  been 
to  give  a  concise  and  connected  account  of  the  work 
which  has  been  carried  out,  particularly  during  the 
last  thirty  years,  with  the  object  of  throwing  light 
on  the  nature  of  the  dyeing  process.  As  will  appear 
from  a  perusal  of  the  book  itself,  many  of  the 
common  practices  of  dyeing  and  the  phenomena 
connected  with  the  process  are  found,  on  examination, 
to  be  in  agreement  with  and  easily  explained  by  the 
general  principles  of  Physical  Chemistry,  with  which 
the  reader  is  supposed  to  be  familiar.  The  book  may 
have,  therefore,  the  effect  of  widening  the  student's 
outlook,  by  showing  him  that  the  principles  which 
govern  many  of  the  operations  of  the  laboratory 
apply  with  equal  force  to  a  large  industry.  Such 
information  respecting  the  textile  fibres  and  the 
dyestuffs  as  is  necessary  for  a  complete  understanding 
of  the  principles  of  dyeing  has  been  included  in  the 
book. 

I  hope  that  the  work  may  also  appeal  to  some  of 
those  actively  engaged  in  the  Dyeing  Industry 
as  well  as  to  the  student,  and  that  it  may  have  the 
effect  of  arousing  in  them  a  greater  interest  in  the 
theoretical  side  of  their  work. 

J.  K.  W. 

DUNDEE,  January  1913. 


f 

603813 


CONTENTS 


I'AUB 


INTRODUCTION         ......  1 

SECTION  I.— THE   CHEMICAL   COMPOSITION   AND  PRO- 
PERTIES   OF   THE    TEXTILE    FlBRES                 .                .  3 

SECTION  II.— DYES  AND  THEIR  PROPERTIES      .           .  12 

SECTION  III. — THE  NATURE  OF  THE  DYEING  PROCESS  30 

BIBLIOGRAPHY        ......  75 

INDEX                                .  79 


Vil 


The  Chemistry  of  Dyeing 


INTRODUCTION 

BY  the  term  "Dyeing"  we  mean  the  colouring  of 
various  materials,  especially  textile  fabrics,  in  such 
a  manner  that  the  colour  is  not  readily  removed  by 
washing  or  rubbing  the  article ;  moreover,  the  colour 
must  be  distributed  right  through  the  whole  material, 
and  not  lie  simply  on  the  surface  as  with  a  painted 
article. 

The  art  of  dyeing  dates  from  prehistoric  times 
and  is  of  Eastern  origin.  Pliny  gives  a  short  account 
of  the  methods  employed  in  Egypt  in  the  first 
century,  but  in  even  earlier  times  dyeing  operations 
were  carried  on  in  India,  China,  and-  Persia.  From 
Egypt  knowledge  of  the  art  travelled  in  a  westward 
direction,  but  it  was  not  until  towards  the  end  of 
the  fifteenth  century  that  the  Dyers'  Company  was 
incorporated  in  London. 

Previous  to  the  middle  of  the  last  century,  all 
the  materials  used  as  colouring  agents  were  of  natural 
origin,  being  chiefly  obtained  from  various  portions 
of  trees  and  plants.  Probably  in  the  early  stages  of 
the  development  of  dyeing,  the  colours  produced  were 
of  a  fugitive  character  and  little  better  than  stains, 

A 


2  THE  CHEMISTRY  OF  DYEING 

but  as '  fame  w£nt.!  pn  methods  were  discovered  by 
means  :  of  .which,  the  colours  could  be  made  more 
permanent*  ;•  'the  .'Egyptians,  for  example,  were  well 
acquainted  with  the  use  of  alum  for  this  purpose. 

In  1856  the  first  artificial  dyestuff  was  manu- 
factured, and  this  marked  the  beginning  of  a  new 
epoch  in  the  dyeing  industry.  During  the  last  fifty 
years  thousands  of  other  artificial  colouring  matters 
have  been  prepared,  with  the  result  that  those  of  a 
natural  origin  have  been  almost  completely  dis- 
placed. The  artificial  dyes  are,  for  the  most  part, 
more  easily  applied  than  the  natural  ones,  as  well  as 
being  more  reliable;  a  much  greater  variety  of 
colour  is  now  possible  than  was  the  case  prior  to  the 
introduction  of  the  artificial  colouring  matters. 

As  regards  the  materials  which  are  dyed,  these 
consist  principally  of  goods  to  be  used  for  clothing, 
upholstery,  etc.,  composed  of  products  of  an  animal 
or  vegetable  origin.  These  animal  and  vegetable 
substances  differ  very  much  in  their  chemical  char- 
acters and  in  their  behaviour  towards  dyes  and  other 
chemicals.  In  addition  to  this,  the  dyes  also  show 
great  diversity  of  constitution  and  properties.  It 
will  be  at  once  apparent,  therefore,  that  before 
studying  the  dyeing  process  and  endeavouring  to 
ascertain  the  nature  of  the  union  between  material 
and  dye,  it  is  necessary  to  be  familiar  with  the' 
properties  of  the  more  important  textile  fibres  and 
with  the  different  groups  of  colouring  matters. 


SECTION  I 

The  Chemical  Composition  and  Properties  of  the 
Textile  Fibres. 

THE  chief  textile  fibres  with  which  the  dyer  comes 
into  contact  are  cotton,  linen,  jute,  wool,  and  silk. 
It  is  customary  to  divide  these  into  two  groups,  the 
first  three  being  classed  as  vegetable  fibres  and  the 
remaining  two  as  animal  ones;  as  will  be  shown 
shortly,  there  are  marked  differences  between  the  two 
kinds  of  fibres. 

A  microscopical  examination  of  the  fibres  reveals 
the  fact  that  they  are  structurally  very  different. 
The  wool  fibre  has  the  most  complex  structure,  being 
made  up  of  cells  of  three  distinct  kinds ;  silk,  on  the 
other  hand,  might  be  said  to  be  devoid  of  structure, 
the  fibre,  as  it  issues  from  the  spinneret  in  the  head 
of  the  worm,  consisting  simply  of  a  long  double 
cylinder.  The  vegetable  fibres  are  composed  of 
hollow  cells,  each  cell  having  a  central  canal  or 
lumen  running  through  it.  In  the  case  of  cotton 
the  fibre  consists  of  a  single  cell,  whilst  with  linen 
and  jute  the  cells  are  grouped  together  in  bundles  to 
form  the  fibres. 

It  is  with  the  differences  of  a  chemical  nature 
•shown  by  the  fibres  that  we  are,  however,  principally 


4  THE  CHEMISTRY  OF  DYEING 

concerned,    for   it   is    largely    upon   these   that   the 
difference  in  the  behaviour  towards  dyestufFs  depends. 

Cotton  and  Linen. 

Cotton  and  linen  consist  essentially  of  the  polysac- 
charose  cellulose,  and  exhibit  in  general  the  properties 
of  that  substance;  instances  of  peculiar  behaviour, 
such  as  that  of  the  cotton  fibre  towards  concentrated 
solutions  of  alkalis,  are  traceable  to  the  structure  of 
the  fibre  and  have  no  connection  with  its  chemical 
composition.  Solutions  of  strong  acids  have  a  hydro- 
lysing  action  on  the  fibres,  which  are  converted 
ultimately  into  dextrin  and  glucose.  Even  with  very 
dilute  solutions  of  such  acids  gradual  disintegration 
of  the  fibre  is  produced  if  the  acid  solution  is  allowed 
to  dry  upon  the  fibre.  Weak  acids  such  as  acetic 
and  formic  acids  have  no  appreciable  action  upon 
cotton  and  linen.  As  is  well  known,  concentrated 
nitric  acid  either  alone  or  in  conjunction  with 
sulphuric  acid  converts  cellulose  into  various  nitrates, 
often  incorrectly  called  nitro-celluloses ;  this  property 
is  now  made  use  of  for  preparing  from  cotton  and 
other  forms  of  cellulose  certain  kinds  of  the  so-called 
artificial  silks  (see  later).  From  the  foregoing  state- 
ments it  will  readily  be  concluded  that  great  care 
must  be  taken  when  dyeing  cotton  and  linen  to 
avoid  the  use  of  dyebaths  containing  considerable 
amounts  of  strong  acids,  whilst  even  when  the 
proportion  of  such  acids  is  very  small  the  treatment 
in  the  bath  must  be  followed  by  thorough  washing 
of  the  material  in  order  to  remove  the  small  amount 
of  acid  present  in  the  cloth,  and  so  prevent  the- 


MERCERISATION  5 

destruction  of  the  fibre  which  would  result  from 
the  gradual  concentration  of  the  acid. 

Dilute  solutions  of  alkalis  have  no  appreciable 
action  on  cotton  or  linen,  but  cold  concentrated  solu- 
tions of  sodium  or  potassium  hydroxide  have  a 
remarkable  effect  upon  the  former  fibre.  The  cotton 
fibre  is  naturally  flat  and  twisted  spirally,  but  after 
treatment  with  the  alkali  it  is  found  to  be  cylindrical 
and  straight.  The  lumen  practically  vanishes  during 
this  treatment,  whilst  the  fibre  becomes  translucent 
and  has  a  superior  attraction  for  colouring  matters 
as  compared  with  the  natural  fibre.  This  behaviour 
of  cotton  was  noticed  first  by  Mercer,  and  is  now 
taken  advantage  of  for  the  manufacture  of  the  so- 
called  mercerised  cotton. 

Solutions  of  hypochlorites,  especially  when  warm, 
convert  cotton  and  linen  into  a  substance  to  which 
Witz  has  given  the  name  of  oxycellulose.  This 
substance  possesses  distinct  acid  properties,  and  has 
a  greater  attraction  for  dyes  of  the  basic  class  than 
have  the  natural  fibres. 

Jute. 

Jute,  although  resembling  cotton  and  linen  in  its 
behaviour  towards  acids,  differs  from  those  fibres  in 
its  chemical  composition.  Cross  and  Bevan,12  who 
have  made  a  study  of  the  subject,  have  given  the 
name  of  bastose  to  the  substance  of  which  jute  is 
composed.  An  insight  into  the  nature  of  this  sub- 
stance is  afforded  by  the  fact  that  on  treatment  with 
alkalis  it  yields  cellulose,  together  with  substances 


6  THE  CHEMISTRY  OF  DYEING 

related  to  the  tannins.  The  researches  of  Cross  and 
Be  van  (loc.  cit.)  have  shown  that  bastose,  which  is 
one  of  the  ligno-celluloses,  may  be  regarded  as  a 
complex  made  up  of  ordinary  cellulose,  a  penta- 
cellulose  containing  an  aldehyde  group  and  yielding 
furf urol  on  hydrolysis,  and  a  substance  of  the  nature 
of  a  quinone,  which,  on  chlorination  and  reduction, 
yields  derivatives  of  the  trihydric  phenols.  Owing  to 
the  presence  of  this  latter  constituent,  jute  behaves 
towards  basic  dyes  in  the  same  manner  as  cotton 
which  has  been  mordanted  with  tannic  acid. 

Wool. 

Wool  and  silk  differ  very  materially  in  their 
chemical  composition  from  the  vegetable  fibres. 
Wool  is  chemically,  as  well  as  structurally,  the  most 
complex  of  the  common  textile  fibres;  it  contains 
nitrogen  and  sulphur  in  addition  to  carbon,  hydrogen, 
and  oxygen.  The  amount  of  sulphur  is  only  small 
and  varies  in  different  samples.  It  appears  probable 
that  this  amount  of  sulphur  does  not  all  enter  into 
the  composition  of  the  fibre  itself,  but  that  the  bulk 
of  it  is  present  as  a  loosely  combined  compound,  since 
most  of  the  sulphur  can  be  removed  from  wool  by 
the  agency  of  alkalis  without  causing  any  apparent 
change  in  the  structure  of  the  fibre.  The  small 
amount  of  the  element  not  so  removed,  and  forming 
about  0-5  per  cent,  of  the  total  weight  of  the  fibre, 
probably  enters  into  the  composition  of  the  wool- 
substance.  The  name  of  keratin  has  been  given  to 
the  substance  of  wjrich  wool  is  composed.  It  is  of 


HYDROLYSIS  OF  WOOL  7 

the  nature  of  a  protein,  and  like  all  such  substances 
is  amphoteric*  in  its  reactions.  Hydrolysis  with 
solutions  of  alkalis  breaks  up  the  keratin  into 
simpler  substances.  One  such  substance  is  lanuginic 
acid,  which  was  isolated  by  Champion,11  and  after- 
wards more  thoroughly  examined  by  Knecht  and 
Appleyard.37  It  is  prepared  by  dissolving  purified 
wool  in  a  moderately  concentrated  solution  of  barium 
hydroxide,  and  then  passing  carbonic  anhydride 
through  the  liquid  to  precipitate  the  barium;  the 
precipitated  barium  carbonate  is  filtered  off  and  the 
filtrate  mixed  with  a  solution  of  lead  acetate.  The 
precipitate  obtained  contains  the  lanuginic  acid;  it 
is  washed,  suspended  in  water,  and  the  lead  thrown 
down  as  sulphide  by  means  of  sulphuretted  hydrogen. 
On  evaporating  the  filtrate  from  the  lead  sulphide 
a  dirty  yellow  residue  remains,  which  is  the  lanuginic 
acid.  Knecht  and  Appleyard  (loc.  cit.)  found  this 
substance  to  give  the  ordinary  protein  reactions; 
it  contains  sulphur  and  dissolves  in  water,  forming 
a  solution  which  is  not  coagulated  by  heat.  Further 

*  An  amphoteric  compound  is  one  which  'is  so  constituted 
that  it  is  capable  of  acting  either  as  an  acid  or  a  base.  The 
simplest  organic  substance  of  this  kind  is  glycine ;  by  virtue 
of  the  carboxyl  group  which  it  contains,  it  is  able  to  react  with 
alkalis  and  form  salts  in  which  it  takes  the  part  of  the  acid, 
while  the  presence  of  the  amino  group  makes  it  possible  for 
glycine  also  to  form  salts  by  union  with  strong  acids,  and  in 
the  latter  case  the  glycine  is  playing  the  part  of  a  base.  Some 
metallic  hydroxides,  such  as,  for  example,  those  of  aluminium, 
zinc,  lead,  etc.,  also  possess  the  power  of  forming  two  classes 
of  salts,  in  one  of  which  they  act  as  a  base  and  in  the  other  as 
an  acid. 


8  THE  CHEMISTRY  OF  DYEING 

reference  to  this  compound  will  be  made  when 
dealing  with  the  theories  of  dyeing. 

The  presence  of  aromatic  amino  groups  in  keratin 
is  indicated  by  the  behaviour  of  wool  towards  nitrous 
acid,  a  diazo  compound  being  formed  which  can  be 
afterwards  coupled  with  a  phenol  in  the  usual 
manner.  The  fact  that  wool,  when  treated  with 
dilute  solutions  of  sulphuric  or  hydrochloric  acid, 
absorbs  and  holds  tenaciously  part  of  the  acid,  is 
further  evidence  of  the  possession  of  basic  properties 
by  the  fibre. 

As  regards  their  effect  upon  the  physical  properties 
of  the  wool  fibre,  acids  are  nothing  like  so  destructive 
as  in  the  case  of  the  vegetable  fibres.  Dilute,  hot 
solutions  of  strong  acids,  even,  have  little  effect  upon 
the  strength  of  the  fibre,  but  concentrated  solutions 
gradually  destroy  it.  Nitric  acid,  as  with  many 
proteins,  turns  wool  yellow  owing  to  the  formation 
of  xanthoproteic  acid. 

On  the  other  hand,  alkalis  are  much  more  severe 
upon  wool  than  upon  cotton  and  linen.  Dilute 
solutions  of  alkaline  hydroxides,  even  in  the  cold, 
weaken  the  fibre,  whilst  on  heating  the  wool 
gradually  dissolves. 

Silk. 

Silk  bears  a  considerable  resemblance  to  wool 
in  its  chemical  properties.  It  is  also  a  protein,  but, 
unlike  wool,  contains  no  sulphur.  Raw  silk  consists 
of  two  substances,  sericin  and  fibroin.  The  former 
constitutes  about  25  per  cent,  of  the  whole;  it  is 


SILK  AND  ITS  PROPERTIES  9 

sometimes  called  silk  glue,  and  has  the  effect  of 
making  the  fibre  stiff  and  harsh  in  feeling.  Before 
being  manufactured  into  silk  goods  the  raw  fibre  is 
scoured  by  means  of  a  solution  of  soap;  this  has 
the  effect  of  removing  the  sericin,  leaving  the 
ordinary  well-known  glossy  fibre  which  consists  of 
fibroin.  Fibroin  gives  the  usual  reactions  of  proteins, 
and  on  hydrolysis  is  resolved  into  a  mixture  of 
amino  acids;  like  keratin  it  contains  the  aromatic 
amino  group. 

Silk  resembles  wool  in  its  behaviour  towards  acids 
and  alkalis,  but  is  rather  less  sensitive  to  the  action 
of  these  substances.  Concentrated  solutions  of 
mineral  acids  and  hot  solutions  of  alkaline  hydroxides 
dissolve  the  fibre. 


Artificial  Silks. 

In  addition  to  the  previously  described  substances, 
some  account  must  be  given  of  certain  artificial 
products  now  in  considerable  use  for  textile  purposes. 
Numerous  attempts  have  been  made  to  prepare 
artificial  products  which  should  have  the  properties 
of  silk  but  be  capable  of  being  produced  at  a  lower 
cost.  Of  these  so-called  artificial  silks,  three  have 
proved  commercially  successful  up  to  the  present. 
The  starting-point  in  the  preparation  of  all  these 
varieties  of  artificial  silk  is  some  form  of  cellulose 
(a  variety  of  artificial  silk  has  been  prepared  from 
gelatin,  the  threads  being  treated  with  formalin  to 
render  them  insoluble,  but  this  process  has  met  with 
no  appreciable  success).  About  half  the  artificial 


10  THE  CHEMISTRY  OF  DYEING 

silk  used  at  the  present  is  prepared  from  the  cellulose 
nitrates;  solutions  of  these  substances  in  alcohol, 
ether,  and  other  solvents  are  squirted  through  capil- 
lary tubes  into  the  air.  A  thread  is  in  this  way 
produced  which  is,  however,  of  too  inflammable  a 
nature  to  be  fit  for  use  in  the  making  of  textiles ;  by 
denitration  with  various  mixtures  a  product  is  ulti- 
mately obtained  which  does  not  exceed  cotton  in 
inflammability.  A  second  process  for  the  preparation 
of  artificial  silk,  giving  rise  to  the  product  known  as 
Glanzstoff,  consists  in  dissolving  cellulose  by  means 
of  an  ammoniacal  solution  of  copper  oxide,  and  forcing 
the  liquid  through  fine  tubes  into  a  solution  of 
sulphuric  acid  or  some  other  coagulating  medium. 

Viscose  silk  is  another  form  of  artificial  fibre  now 
being  manufactured  in  considerable  quantities.  Cellu- 
lose is  treated  with  a  15  per  cent,  solution  of  sodium 
hydroxide,  and  the  resulting  mass,  after  being  squeezed, 
is  then  submitted  in  a  closed  vessel  to  the  action  of 
carbon  bisulphide.  After  several  hours  a  product 
is  obtained  which  is  known  as  viscose,  from  the  fact  that 
when  dissolved  in  water  it  gives  rise  to  an  extremely 
viscous  liquid.  By  squirting  this  product  through 
platinum  jets  into  solutions  of  ammonium  salts  threads 
are  obtained  which  can  be  made  up  into  textiles. 

It  will  be  evident  from  the  description  which  has 
been  given  of  the  methods  of  preparation  of  artificial 
silk,  that  these  products  are  chemically  more  allied 
to  cotton  than  to  silk.  Saget  and  Silvern  have  stated 
that  whereas  natural  silk  contains  about  17  per  cent, 
of  nitrogen,  the  various  makes  of  artificial  fibre  all 
contain  less  than  0-25  per  cent,  of  that  element. 


ARTIFICIAL  SILKS  11 

Consisting  essentially  of  cellulose,  the  artificial  silks 
behave  towards  reagents  in  the  same  way  as  cotton 
does.  The  artificial  fibres  also,  for  the  most  part, 
resemble  cotton  in  their  behaviour  towards  dyestuifs, 
the  principal  point  of  difference  being  that  the  silk 
from  the  cellulose  nitrates  can  be  dyed  directly  with 
dyes  of  the  basic  group. 


SECTION   II 

Dyes  and  their  Properties. 

As  was  stated  in  the  introduction,  thousands  of 
artificial  colouring  matters  are  now  known.  It  is 
not  within  the  scope  of  the  present  work  to  enter 
into  a  full  consideration  of  the  constitution  and 
methods  of  preparation  of  the  colouring  matters ;  for 
information  on  these  subjects  the  reader  may  be 
referred  to  Cain  and  Thorpe's  work  on  The  Synthetic 
Dyestuffs. 

It  has  been  found  that  these  substances  belong  to 
certain  classes  of  organic  compounds,  so  that  it  is 
evident  that  the  question  of  constitution  enters  into 
the  problem  as  to  whether  a  particular  substance 
shall  be  a  dyestuff  or  not.  Witt  came  to  the  con- 
clusion that  every  dyestuff  must  contain  one  or  more 
of  certain  groups  which  determined  the  character  of 
the  dye,  and  which  he  called  chromophores.  Amongst 
such  groups  may  be  mentioned  the  azo  group 
—  N  =  N  — ,  the  para  quinone  group  =<^  \=,  etc. 
The  simplest  substance  containing  a  chromophore 
was  called  a  chronaogen.  In  the  case  of  the  azo 
group,  azo-benzene  would  be  the  chromogen.  Azo- 


CLASSIFICATION  OF  DYES  13 

benzene,  however,  though  highly  coloured,  is  not 
a  dyestuff,  and  cannot  be  used  for  the  dyeing  of 
textile  fabrics.  The  presence  of  a  second  group, 
called  an  auxochrome  group,  is  necessary  before  the 
chromogen  can  become  a  dyestuff.  The  auxochrome 
confers  salt-forming  properties  on  the  compound, 
and  is  in  general  either  the  amino  or  the  phenolic 
group.  It  will  be  seen,  therefore,  that  all  substances 
which  act  as  dyestuffs  must  possess  either  basic  or 
acidic  properties. 

From  the  point  of  view  of  the  dyer  it  is  much 
better  to  classify  the  colouring  matters  according 
to  their  methods  of  application  to  the  textile  fibres 
rather  than  according  to  their  chemical  constitution, 
although,  of  course,  the  method  of  application  is 
dependent  on  the  constitution.  When  the  practical 
method  of  classification  is  adopted  it  is  found  that 
the  dyes  fall  into  six  or  seven  different  groups. 

The  Basic  Dyes,  as  their  name  indicates,  are  basic 
compounds,  and  are  employed  in  combination  with 
hydrochloric  acid  or  zinc  chloride.  Dyes  of  this 
class  can  be  applied  directly  to  wool  and  silk,  but 
an  acid  mordant  such  as  tannic  acid  or  a  fatty  acid 
is  necessary  in  order  to  fix  them  upon  cotton  or  linen. 
Jute,  owing  to  its  different  chemical  composition, 
behaves  towards  dyes  of  this  group  differently  from 
cotton,  and  can  be  dyed  directly.  Many  derivatives 
of  triphenyl-methane  (such  as  Magenta,  Methyl 
Violet,  Malachite  Green,  etc.),  and  similar  substances, 
amino-azo  compounds,  acridine  derivatives  and  certain 
phthaleins,  such  as  Rhodamine,  belong  to  this  group 
of  colouring  matters.  It  must  be  noted  that  the 


14  THE  CHEMISTRY  OF  DYEING 

introduction  of  the  sulphonic  group  into  any  of  the 
above  types  of  compound  destroys  the  basic  character 
of  the  dye. 

The  Acid  Dyes  consist  of  the  sodium  salts  of 
sulphonic  acids  of  all  kinds.  Dyes  of  this  group 
are  of  particular  importance  for  the  dyeing  of  the 
animal  fibres,  to  which  the  colouring  matters  can 
be  applied  directly  without  the  use  of  a  mordant 
being  necessary.  The  acid  dyes  are  seldom  used  for 
the  dyeing  of  the  vegetable  fibres;  when  employed 
for  this  purpose,  a  basic  mordant  such  as  alum  is 
required.  Nitrophenols  also  belong  to  this  class  of 
dyes. 

The  Direct  Cotton  Dyes  form  a  very  important 
group  of  colouring  matters,  since  they  possess  the 
property  of  dyeing  cotton  and  linen,  as  well  as  wool 
and  silk,  without  requiring  the  aid  of  a  mordant. 
They  are  salts,  and  are  azo  compounds  derived  from 
diamines  such  as  benzidine,  tolidine,  etc. 

A  number  of  other  colouring  matters  capable  of 
direct  application  to  cotton  have  come  into  extensive 
use  of  late  years.  These  are  the  dyes  known  as 
sulphur  or  sulphide  colours,  and  they  are  prepared 
by  fusing  certain  aromatic  amines,  etc.,  with  sulphur 
and  sodium  sulphide.  The  products  obtained  are  in- 
soluble in  water  and  cannot  be  purified  by  recrystal- 
lisation,  so  that  the  actual  composition  of  these  dyes 
cannot  be  readily  determined ;  probably  the  majority 
of  these  colouring  matters  consist  of  complex  mixtures. 
Although  insoluble  in  water,  the  dyes  dissolve  in  a 
solution  of  sodium  sulphide,  and  the  solution  to  be 
used  in  the  dyebath  is  prepared  in  this  manner, 


CLASSIFICATION  OF  DYES  15 

sodium  sulphate  and  soda  ash  being  added  as  assistants. 
It  has  been  supposed  that  the  solution  contains  the 
dye  in  the  form  of  its  leuco  compound,  the  actual 
colouring  matter  being  reformed  on  exposure  to  the 
air. 

Mordant  Dyes. — In  distinction  from  the  last  group 
of  colouring  matters,  we  have  a  fourth  class  known 
as  the  mordant  dyes,  which  are  distinguished  from 
other  dyestuffs  by  the  fact  that  the  use  of  a  mordant 
is  invariably  necessary  in  order  to  fix  the  dye,  no 
matter  what  the  nature  of  the  material  of  which 
the  fabric  is  composed.  The  mordants  usually  used 
are  salts  of  aluminium,  chromium,  and  iron.  The 
majority  of  the  natural  dyestuffs  belong  to  this  class, 
and  many  valuable  wool  dyestuffs  also  belong  to  it. 
They  are  of  an  acidic  character,  and  contain  phenolic 
or  carboxyl  groups.  The  most  important  dye  of  this 
group  is  probably  Alizarin,  which  is  used  very 
extensively  for  the  dyeing  of  cotton  as  well  as  being 
employed  in  wool  dyeing. 

Vat  Colours. — Another  group  of  colouring  matters 
form  the  class  known  as  the  vat  colours.  Up  to 
a  few  years  ago  Indigo  was  practically  the  only 
member  of  this  group,  but  during  the  last  few  years 
a  number  of  other  dyestuffs  have  been  prepared,  all 
of  which  are  applied  in  the  same  way  as  the  important 
colouring  matter  mentioned  above. 

The  dyestuffs  of  this  class  are  all  capable  of  reduction, 
and  it  is  the  reduction  product  which  is  actually 
absorbed  by  the  fabric.  Some  of  the  dyes  of  this 
group,  as,  for  example,  the  Algole  and  Indanthrene 
colours,  are  only  suitable  for  cotton,  as  they  require 


16  THE  CHEMISTRY  OF  DYEING 

a  strongly  alkaline  bath  for  their  application.  The 
solutions  of  the  reduction  products  of  dyes  of  this 
character  are  darker  in  colour  than  the  original 
substance. 

The  Indigo,  Thio-indigo,  Helindone,  and  Ciba  dyes, 
on  the  other  hand,  only  require  at  the  most  a  slightly 
alkaline  bath,  and  can  therefore  be  applied  to  the 
animal  as  well  as  to  the  vegetable  fibres;  the 
solutions  of  the  reduced  dyes  are  of  a  lighter  colour 
than  the  original  compounds.  After  the  leuco  com- 
pound has  been  absorbed  by  the  fabric  the  original 
substance  is  reformed  in  the  pores  of  the  fibre  by 
exposing  the  wet  material  to  the  oxidising  influence 
of  the  atmosphere. 

Developed  Dyes. — The  last  group  of  colouring 
matters  to  be  mentioned  may  be  described  as  the 
developed  dyes,  since  they  are  actually  formed  on 
the  fibre  by  the  interaction  of  the  substances  necessary 
for  their  preparation.  The  so-called  mineral  colouring 
matters,  such  as  Chrome  Yellow,  belong  to  this  group, 
the  colour  just  alluded  to  being  formed  within  the 
pores  of  the  fibre  as  a  result  of  the  interaction  of 
a  lead  compound  with  a  solution  of  a  bichromate. 

Aniline  Black,  a  well-known  and  very  fast  colouring 
matter  for  cotton,  is  also  included  in  this  class,  as 
it  is  formed  by  oxidising  an  aniline  salt  with  which 
the  fabric  has  been  impregnated.  (In  the  more 
modern  method  the  cotton  is  not  first  treated  with 
the  aniline  salt,  but  is  immersed  directly  in  a  bath 
containing  the  aniline  salt  and  the  oxidising  agent; 
on  heating  the  mixture  the  colouring  matter  is  formed 
and  deposited  on  the  fibre.) 


CLASSIFICATION  OF  DYES  17 

A  number  of  azo  dyes  are  also  developed  on  the 
fibre;  these  are  sometimes  known  as  ice  colours 
because  of  the  fact  that  ice  is  frequently  necessary 
to  prevent  the  decomposition  of  the  diazo  compound, 
which  is  one  of  the  reacting  substances.  The 
substance  applied  to  the  fabric  may  be  a  dyestuff 
itself  or  not;  the  only  essential  is  that  it  must 
contain  a  primary  aromatic  amino  group  and  so  be 
capable  of  diazotisation.  After  the  treatment  with 
nitrous  acid  the  colour  is  developed  by  treating  the 
material  with  a  solution  of  a  phenol  or  an  amine, 
when  coupling  takes  place  in  the  usual  manner  with 
the  diazo  compound. 

Many  of  the  direct  cotton  dyes  which  are  not 
themselves  very  fast  can  be  converted  by  diazotising 
and  developing  into  very  fast  products.  One  of  the 
most  useful  developers  is  /3-naphthol.  Of  the  amino 
compounds  employed  in  this  manner  which  are  not 
themselves  dyes,  mention  may  be  made  of  _p-nitrani- 
line,  a-naphthylamine,  benzidine,  etc. ;  when  these 
substances  are  made  use  of  the  phenol  is  usually 
applied  to  the  cloth,  which  is  then  introduced  into 
the  cooled  solution  of  the  diazotised  amine. 


Application  of  the  Dyes. 

As  regards  the  methods  of  application  of  the  various 
types  of  dyestuffs  to  the  different  fibres,  these  have 
been  sufficiently  indicated  in  principle  in  the  fore- 
going pages.  Details  as  to  the  mode  of  working  in 
the  case  of  individual  dyes  are  best  obtained  from 
the  pattern  books  and  notices  issued  by  all  the 

B 


18  THE  CHEMISTRY  OF  DYEING 

leading  manufacturers  of  dyestuffs.  The  principles 
underlying  certain  of  the  practices  commonly  followed 
may,  however,  be  discussed  here. 

It  is  a  common  practice  to  add  to  the  dyebath 
a  substance  known  as  an  assistant.  Two  of  the 
most  commonly  used  assistants  are  sulphuric  acid 
and  sodium  sulphate,  while  in  the  dyeing  of  silk 
boiled-off  liquor  or  soap  is  frequently  added  to  the 
bath.  Sulphuric  acid  is  used  in  conjunction  with 
the  acid  dyestuffs.  It  acts  upon  the  dyestuff  which, 
as  has  been  stated,  is  a  salt  of  a  sulphonic  acid,  and 
liberates  the  free  colour  acid  which  is  the  substance 
probably  taken  up  by  the  fibre.  The  added  acid 
(this  is  not  necessarily  sulphuric;  acetic  and  formic 
acids  are  employed  to  a  considerable  extent  in  this 
connection)  also  has  the  effect  of  diminishing  the 
solubility  of  the  colour  acid.  This  is  because  of  the 
fact  that  the  two  substances  give  rise  to  a  common 
ion,  hydrion,  and  the  law  of  mass  action  applies  to 
this  as  to  all  such  cases. 

The  question  of  the  amount  of  acid  to  be  added 
depends  on  the  character  of  the  dye.  The  more 
rapidly  a  colouring  matter  is  absorbed  by  a  fibre, 
the  greater  is  the  danger  of  the  dyed  material  being 
uneven  in  colour.  With  a  dyestuff,  therefore,  which 
is  readily  absorbed  by  wool,  an  excess  of  acid  is  to  be 
avoided,  so  as  to  prevent  the  solubility  of  the  colour- 
acid  from  being  sensibly  diminished;  too  rapid 
absorption  of  the  dye  will  in  this  way  be  obviated. 
The  same  end  can  be  attained  by  an  addition 
of  sodium  sulphate  in  conjunction  with  the  sul- 
phuric acid;  the  sodium  hydrogen  sulphate  formed 


THE  USES  OF  ASSISTANTS  19 

from  the  two  substances  is  a  weaker  acid  than  the 
sulphuric  acid  itself,  and  has  therefore  less  action 
upon  the  dyestuff,  as  it  cannot  compete  so  strongly 
for  the  base  of  the  dyestuff,  which  means  that  a 
smaller  amount  of  colour  acid  will  be  liberated. 
Owing  also  to  sodium  hydrogen  sulphate  being  a 
weaker  acid  than  sulphuric  acid,  an  excess  of  the 
former  has  much  less  effect  on  the  solubility  of  the 
colour  acid  than  is  produced  by  an  excess  of  sulphuric 
acid.  A  too  rapid  deposition  of  colouring  matter  is 
also  avoided  by  working  at  a  low  temperature. 

When  acid  dyes  are  employed  for  dyeing  silk  an 
acid  is  again  added  to  the  bath  for  the  same  reasons 
as  have  been  mentioned  in  the  case  of  wool,  and  the 
same  considerations  are  necessary  in  determining  the 
quantity  of  acid  to  be  added.  When  it  is  necessary 
to  regulate  the  rate  at  which  the  dyestuff  is  absorbed, 
an  addition  of  soap  or  of  boiled-off  liquor  may  be 
made  instead  of  sodium  sulphate. 

As  regards  the  other  classes  of  dyestuffs,  the 
members  of  the  direct  cotton  group  are  the  ones  for 
which  the  use  of  an  assistant  is  most  necessary.  The 
dyes  of  this  group,  it  has  been  already  stated,  have 
the  character  of  salts,  and  it  is  customary  to  add 
along  with  them  to  the  dyebath  some  other  salt, 
such  as  sodium  sulphate  or  chloride.  The  effect 
of  this  addition  is  exactly  similar  to  that  caused 
by  the  addition  of  an  excess  of  acid  to  the  acid  dyes, 
that  is,  by  increasing  the  active  mass  of  one  of  the 
ions  to  which  the  dyestuff  gives  rise,  the  solubility 
of  the  dye  is  diminished.  Too  rapid  deposition  of 
the  dye  upon  the  fibre  is  in  this  case  prevented  by 


20  THE  CHEMISTRY  OF  DYEING 

adding  a  substance  of  an  alkaline  character,  such  as 
soda  ash,  along  with  the  sodium  sulphate,  the  alkali 
tending  to  keep  the  colouring  matter  in  solution. 
When  silk  goods  are  being  dyed,  the  sodium  sulphate 
is  replaced  by  boiled-off  liquor  or  soap. 

No  addition  is  made  to  the  bath  with  the  basic 
dyes  unless  it  is  desired  to  retard  the  rate  of  absorp- 
tion, in  which  case  a  small  amount  of  sulphuric  or 
other  acid  is  added ;  this  has  a  solvent  action  on  the 
dye  and  brings  about  the  desired  effect. 

The  Condition  of  Dyes  in  Solution. 

Of  late  years  a  considerable  amount  of  work  has 
been  done  in  connection  with  the  mode  of  existence 
of  dyestuffs  in  aqueous  solution,  and  as  the  subject 
is  of  interest  because  of  its  bearing  on  the  principles 
underlying  the  dyeing  process,  an  account  of  the  more 
important  work  in  this  direction  will  now  be  given. 

As  has  been  indicated  in  the  earlier  part  of  the 
section,  the  dyes  of  the  acidic,  basic,  and  direct 
cotton  groups  are  all  of  the  nature  of  salts,  and 
assuming,  therefore,  that  they  form  true  solutions 
when  added  to  water,  they  should  be  largely  ionised, 
diffuse  through  a  parchment  membrane  and,  in  fact, 
exhibit  all  the  phenomena  of  salts  in  general ;  probably 
in  some  cases  where  the  acid  or  the  base,  or  both  of 
these,  is  weak,  a  certain  amount  of  hydrolysis  would 
take  place.  It  will  be  evident  that  a  great  deal 
depends  on  whether  the  dyes  exist  in  a  state  of 
true  solution  or  not,  and  most  of  the  recent  workthas 
been  directed  to  the  settlement  of  this  problem. 


DIALYSIS  OF  DYE-SOLUTIONS  21 

Amongst  the  earliest  observations  dealing  with  this 
subject  may  be  mentioned  those  of  Pfeffer,47  who 
noticed  that  some  of  the  basic  colouring  matters,  such 
as  Methylene  Blue  and  Methyl  Violet,  are  able  to 
pass  through  the  wall  of  a  living  cell  and  colour 
the  protoplasm.  Obviously,  therefore,  dyes  such  as 
those  mentioned  must  exist  in  true  solution,  for  unless 
such  were  the  case  no  dialysis  could  take  place. 
Later  investigations  go  to  show  that  this  property 
of  existing  in  a  state  of  true  solution  is  not  one 
which  applies  to  dyes  in  general. 

True  and  Colloidal  Solutions. — By  means  of 
dialysis  experiments  and  also  as  a  result  of  examining 
solutions  with  the  ultra-microscope,  Freundlich  and 
Neumann22  have  come  to  the  conclusion  that  dye- 
stuffs  should  be  divided  into  three  classes,  and  a 
similar  conclusion  was  arrived  at  by  Biltz  and 
Pfenning.6 

The  first  group  consists  of  those  colouring  matters 
which  diffuse  readily  through  parchment  paper,  and 
which  are  therefore  present  in  true  solution ;  amongst 
the  dyes  of  this  class  may  be  mentioned  Chrysoidine, 
Bismarck  Brown,  Auramine,  Eosine,  Methylene  Blue, 
Safranine,  Picric  Acid,  etc. 

The  second  group  contains  dyes  which  form  solu- 
tions of  a  semi-colloidal  nature,  that  is  to  say,  dialysis 
takes  place,  but  at  a  very  slow  rate.  Magenta,  Methyl 
Violet,  Capri  Blue,  and  Nile  Blue  are  a  few  of  the 
dyes  which  fall  into  this  group. 

The  third  class  contains  the  dyes  which  form 
colloidal  solutions  proper,  as  indicated  by  their  non- 
diffusibility  through  a  membrane,  and  by  the  fact 


22  THE  CHEMISTRY  OF  DYEING 

that  the  solutions,  when  examined  with  the  ultra- 
microscope,  are  not  optically  void.  Many  of  the 
dyes  of  the  direct  cotton  group,  such  as  Congo  Red, 
Benzopurpurin,  Benzoazurin,  etc.,  are  of  this  type, 
but  the  property  of  forming  colloidal  solutions  is  not 
confined  to  the  direct  cotton  colours;  Night  Blue, 
Induline,  Alkali  Blue,  etc.,  all  form  solutions  of  this 
nature. 

Freundlich  and  Neumann  (loc.  cit.)  divide  the  dyes 
of  the  third  group  into  two  sections,  according  to 
whether  they  have  a  marked  influence  or  not  on  the 
properties  of  the  solvent.  The  first  section  contains 
the  dyes  which  form  solutions  of  the  same  nature  as 
that  given  by  arsenious  sulphide.  They  are  called 
"  suspensoids,"  and  are  precipitated  by  the  addition 
of  small  quantities  of  salts.  As  regards  the  dyes  of 
the  other  section,  called  "emulsion  colloids,"  they 
require  the  addition  of  considerable  amounts  of  salts 
before  precipitation  takes  place,  and  influence  very 
considerably  the  surface  tension  and  other  physical 
properties  of  the  solvent;  the  solutions  formed  by 
dyes  of  the  latter  type  bear  a  resemblance  to  solutions 
of  gelatine.  Night  Blue  is  a  colloid  of  the  latter  kind, 
while  Congo  Bed  is  one  of  the  colouring  matters 
included  in  the  first  sub-section. 

Molecular  Complexity  of  Dyestuffs. — Biltz  and 
Pfenning  (loc.  cit.)  have  drawn  the  conclusion  that 
the  dialysing  power  of  a  dyestuff  depends  on  its 
molecular  complexity.  If  the  molecule  of  the  dye 
contains  less  than  forty-five  atoms  the  substance  will 
diffuse  rapidly  through  parchment  or  through  a 
collodion  membrane,  but  as  the  number  of  atoms  in 


DIALYSIS  OF  DYE-SOLUTIONS  23 

the  molecule  increases,  the  rate  of  diffusion  diminishes ; 
with  dyes  containing  between  fifty-five  and  seventy 
atoms  in  the  molecule  the  velocity  of  dialysis  is 
very  small,  while  when  the  number  of  atoms  exceeds 
seventy  dialysis  ceases  altogether. 

This  relationship  between  the  complexity  of  the 
substance  and  the  rate  of  dialysis  is,  however, 
influenced  to  a  certain  extent  by  the  composition 
and  constitution  of  the  substance.  It  was  found, 
for  example,  that  the  introduction  of  the  sulphonic 
acid  group  into  a  compound  has  a  marked  effect  in 
increasing  the  dialysing  capacity  of  the  substance; 
dyes  of  the  Malachite  Green  series  containing  two 
or  three  sulphonic  acid  groups  pass  readily  through 
a  membrane  even  when  the  number  of  atoms  in  the 
molecule  exceeds  seventy.  Dyes  having  a  constitution 
allied  to  that  of  Alizarin,  on  the  other  hand,  dialyse 
less  readily  than  would  be  expected  from  the  number 
of  atoms  contained  in  the  molecule. 

Pelet- Jolivet  and  Wild 45  agreed  with  the  previously 
mentioned  investigators  in  coming  to  the  conclusion 
that  dyes  can  be  divided  into  three  classes  according 
to  the  character  of  the  solutions  they  form,  but  did 
not  always  agree  as  to  the  class  in  which  certain 
dyes  should  be  placed.  Their  methods  of  investigation 
included  determinations  of  electrical  conductivity  and 
the  use  of  the  ultra-microscope.  They  also  determined 
the  effect  of  dye  solutions  upon  diazo  acetic  ester 
and  noticed  no  catalytic  action,  from  which  they 
naturally  concluded  that  the  colouring  matters 
examined  did  not  undergo  hydrolysis  in  aqueous 
solution.  This  is  opposed  to  the  views  commonly 


24  THE  CHEMISTRY  OF  DYEING 

held,  and  it  would  be  of  interest  to  ascertain  whether 
the  results  can  be  confirmed.  Biltz  and  Pfenning7 
have  made  determinations  of  the  electrical  conductivity 
and  osmotic  pressure  of  solutions  of  dyestuffs  which 
had  been  freed  from  admixed  inorganic  salts  by  the 
process  of  dialysis.  The  conductivity  results  seem 
to  show  that  the  substances  examined  behave  as 
normal  electrolytes,  although  they  do  not  obey 
Ostwald's  rule  regarding  the  dependence  of  the 
molecular  conductivity  on  the  basicity  of  the  acid. 
The  results  of  the  determinations  of  osmotic  pressure 
showed  that  a  dye  solution  has  in  many  cases  a 
complex  character,  as  products  of  association,  ionisa- 
tion,  and  hydrolysis  may  be  existing  together  in  a 
state  of  equilibrium.  With  solutions  of  mono- 
sulphonates  the  association  and  hydrolysis  products 
are  the  most  important,  and  the  molecular  weight 
calculated  from  the  osmotic  pressure  is  much  higher 
than  the  theoretical  value.  With  the  disulphonic 
derivatives  ionisation  practically  balances  association, 
so  that  the  molecular  weights  which  are  determined 
agree  on  the  whole  with  the  theoretical  values. 
When  the  number  of  sulphonic  acid  groups  exceeds 
two,  the  question  of  ionisation  becomes  of  most 
importance,  and  owing  to  the  extent  to  which  this 
takes  place  the  observed  values  for  the  molecular 
weights  are  considerably  smaller  than  the  theoretical 
values. 

Determinations  of  the  electrical  conductivity  of 
solutions  of  dyestuffs  have  also  been  made  by  Knecht 
and  Batey,38  some  of  the  dyes  experimented  with,  as 
for  example,  Benzopurpurin,  being  amongst  those 


EXPERIMENTS  WITH  CONGO  RED  25 

classed  as  colloids  by  other  investigators.  They 
found  the  solutions  to  be  good  conductors,  and  the 
results  obtained  with  dilute  solutions  were  such  as 
to  indicate  a  high  degree  of  ionisation.  The  results 
of  the  conductivity  experiments  were  supported  by 
the  values  obtained  for  the  molecular  weights  of 
the  dyes  by  the  boiling-point  method.  With  Benzo- 
purpurin,  Soluble  Blue,  and  Chrysophenine,  the 
observed  elevation  of  boiling-point  of  the  aqueous 
solution  was  such  as  to  indicate  a  considerable  amount 
of  ionisation.  The  authors  mentioned  also  found 
Benzopurpurin  to  have  a  high  rate  of  dialysis,  and 
they  are  consequently  opposed  to  the  view  that  it 
and  many  other  dyestuffs,  especially  those  of  high 
molecular  weight,  exist  in  the  state  of  colloidal 
solution. 

Results  of  an  interesting  and  important  character 
have  been  obtained  from  the  investigation  of  solutions 
of  Congo  Red  and  allied  colouring  matters.  Bayliss  3 
showed  that  although  this  colouring  matter  will  not 
pass  through  a  parchment  membrane  and  also 
exhibits  other  properties  associated  with  colloids, 
yet  it  exerts  an  osmotic  pressure  equal  to  that  which 
would  be  expected  if-  it  was  present  in  a  state  of 
true  solution  in  the  unassociated  form.  This  result 
was  only  obtained,  however,  when  the  outside  vessel 
was  filled  with  distilled  water;  if  solutions  of  acids, 
bases,  or  salts  were  used  instead  of  water,  the  values 
of  the  osmotic  pressure  determined  were  of  consider- 
ably lower  magnitude.  Bayliss  considered  the  electro- 
lyte to  have  the  effect  of  causing  the  molecules  of 
dye  to  collect  together  to  form  aggregated  particles. 


26  THE  CHEMISTRY  OF  DYEING 

Similar  experiments  were  carried  out  by  Biltz  and 
Vegesack,8  making  use  of  Congo  Red,  Benzopurpurin, 
and  Night  Blue.  The  results  obtained  with  Congo 
Ked,  previously  freed  from  admixed  salts  by  pro- 
longed dialysis,  confirmed  those  obtained  by  Bayliss. 
With  water  in  the  outer  vessel  the  value  calculated 
for  the  molecular  weight  was  602,  the  theoretical 
value  being  696,  whilst  the  result  obtained  when 
the  outer  vessel  was  filled  with  a  salt  solution  of 
the  same  conductivity  as  the  dye  solution  was  2333. 
From  a  consideration  of  the  equilibrium  between 
the  ions  of  Congo  Red  and  those  of  sodium  sulphate, 
the  conclusion  was  drawn  that  if  the  solute  is  not 
polymerised  the  apparent  molecular  weight  of  Congo 
Red  should  be  three  times  the  normal  value,  that 
is,  2088 ;  as  this  figure  is  only  slightly  different  from 
the  one  actually  determined,  Biltz  and  Vegesack 
came  to  the  conclusion  that  Congo  Red  only  under- 
goes slight  polymerisation  in  solution.  Results  of 
a  similar  character  were  obtained  with  Night  Blue, 
but  in  this  case  complications  were  introduced  because 
of  the  fact  that  a  certain  amount  of  hydrolysis  takes 
place. 

The  investigation  was  extended  to  the  examination 
of  commercial  preparations  of  certain  dyestuffs. 
Many  of  these  products  contain  a  certain  quantity 
of  salts,  chiefly  sodium  sulphate;  in  Congo  Red 
the  amount  of  impurity  is  about  26  per  cent.  The 
molecular  weight  of  Congo  Red,  as  determined  from 
this  sample,  was  found  to  be  about  7380,  from 
which  the  conclusion  was  drawn  that  association 
takes  place  in  the  presence  of  salts. 


EXPERIMENTS  WITH  CONGO  RED  27 

This  view  of  the  effect  of  electrolytes  on  the 
molecular  complexity  of  dyes  such  as  Congo  Eed, 
Night  Blue,  and  Benzopurpurin,  received  additional 
support  from  ultra-microscopic  examinations  of  the 
solutions.  The  effect  of  the  electrolyte  appears  to 
become  more  pronounced  when  the  solution  is  kept, 
and  the  polymerisation  is  greater  in  concentrated 
than  in  dilute  solutions.  The  temperature  of  the 
solution  also  appears  to  have  a  marked  influence  on 
the  degree  of  association;  Biltz  and  Vegesack,  from 
the  determination  of  the  osmotic  pressure  at  different 
temperatures,  came  to  the  conclusion  that  in  a  solution 
of  Night  Blue  the  degree  of  association  at  0°  was 
6-7,  at  25°  it  was  3-05,  while  at  70°  the  value  was 
only  1-9. 

Anomalous  Behaviour  of  Dyestuffs  on  Dialysis. 
— An  important  paper  dealing  with  this  subject  was 
published  recently  by  Donnan  and  Harris.15  Several 
of  the  observations  they  made  confirmed  those  of 
the  previous  workers  on  this  subject,  as,  for  example, 
that  Congo  Red  gives  an  osmotic  pressure  when 
measured  against  distilled  water  which  agrees  approxi- 
mately with  the  value  which  would  be  obtained  for 
a  true  solution  in  which  the  dye  was  present 
as  single  molecules.  From  the  determinations  of 
electrical  conductivity  they  drew  the  conclusion  that 
the  dye  exists  to  a  very  considerable  extent  in  the 
ionised  condition. 

Amongst  the  most  important  of  their  observations 
were  those  dealing  with  the  dialysis  of  solutions  of 
dyes  of  the  Congo  type.  It  was  found  that  both  with 
Congo  Red  and  Benzopurpurin  4B  a  peculiar 


28  THE  CHEMISTRY  OF  DYEING 

"  membrane  hydrolysis "  takes  place,  sodium  ions  in 
company  with  hydroxyl  ions  diffusing  out  of  the 
dialyser,  while  the  free  dye  acid,  an  acid  salt,  or  some 
other  insoluble  phase  remains  behind.  It  was  found 
possible  to  prevent  this  hydrolysis  by  adding  to  the 
dye  solution  an  alkaline  hydroxide,  the  quantity  of 
alkali  required  depending  on  the  temperature  and 
concentration  of  the  solution  of  colouring  matter.  It 
is  evident,  therefore,  in  the  light  of  these  observations, 
that  the  osmotic  pressure  observed  with  a  solution  of 
a  colouring  matter  like  Congo  Red  does  not  corre- 
spond with  the  ordinary  state  of  osmotic  equilibrium. 
Donnan  and  Harris  also  confirmed  the  fact  that 
the  osmotic  pressure  of  Congo  Red  is  lowered  in  the 
presence  of  certain  electrolytes,  but  they  threw  a 
considerable  amount  of  new  light  on  this  matter,  and 
were  led  to  conclusions  which  must,  if  they  are 
accepted,  and  the  author  sees  no  reason  for  adopting 
any  other  attitude,  lead  to  the  abandonment  of  some 
of  the  views  previously  held. 

It  was  found  that  when  Congo  Red  and  sodium 
chloride  were  introduced  into  a  dialyser,  the  sodium 
chloride  distributed  itself  unequally  on  the  two  sides 
of  the  parchment  membrane,  and  a  reversible  ionic 
equilibrium  was  ultimately  established,  the  concentra- 
tion of  sodium  chloride  at  equilibrium  being  higher 
on  the  side  opposite  to  that  occupied  by  the  dyestuff. 
It  was  found  that  this  membrane  equilibrium  is 
necessary  from  thermodynamical  considerations,  and 
that  it  is  one  of  a  group  of  phenomena  of  a  general 
character.  It  is  obvious  that  because  of  the  unequal 
distribution  of  the  salt  on  the  two  sides  of  the 


EXPERIMENTS  WITH  CONGO  RED  29 

membrane  an  opposed  osmotic  pressure  will  be  set 
up,  and  this  is  probably  the  cause  of  the  smaller 
values  obtained  with  solutions  of  dyes  containing 
dissolved  salts,  or  where  the  outer  vessel  contains 
salts. 

All  the  conclusions  which  have  been  drawn,  there- 
fore, regarding  the  molecular  complexity  of  dyes  in 
solution  from  measurements  of  the  osmotic  pressure 
are  valueless  when  another  electrolyte  is  present, 
unless  account  be  taken  of  the  unequal  distribution 
of  the  foreign  electrolyte.  The  subject  is  one  of 
great  interest,  and  no  doubt  further  experiments  will 
be  made  in  the  near  future  which  will  shed  additional 
light  on  the  condition  of  dyestuffs  in  solution. 

While  at  present,  in  view  of  the  results  obtained 
by  Donnan  and  Harris,  it  would  be  unwise  to  lay  too 
much  stress  upon  the  results  of  dialysis  experiments 
with  solutions  of  dyes,  it  is  yet  quite  clear  from  the 
evidence  gained  by  the  use  of  the  ultra-microscope 
that  many  colouring  matters  in  aqueous  solution 
exist  to  a  greater  or  lesser  extent  in  the  colloidal 
condition.  On  the  other  hand,  the  aqueous  solutions 
of  other  dyes  are  optically  void,  and  appear,  from 
determinations  of  the  electrical  conductivity,  to  be 
ionised  to  a  considerable  extent;  in  such  cases  we 
are  justified  in  believing  that  the  dye  is  present 
in  a  state  of  true  solution. 


SECTION   III 

The  Nature  of  the  Dyeing  Process. 

IT  is  not  surprising  that  in  the  case  of  an  industry 
so  old  and  so  universal  as  is  that  of  Dyeing  many 
speculations  should  have  been  made  as  to  the  nature 
of  the  process  by  which  the  colouring  matter  becomes 
fixed  on  the  fibre  in  an  insoluble  form.  An  attempt 
will  be  made  in  the  following  pages  to  give  some 
account  of  the  different  ideas  which  have  from  time 
to  time  been  held  on  the  subject,  together  with  the 
experimental  evidence  on  which  these  theories  have 
been  based,  and  the  criticisms  which  have  been 
brought  against  them.  As  can  readily  be  imagined 
when  one  considers  the  diversity  of  substances 
involved,  both  the  dyes  and  the  fibres,  the  subject  is 
one  fraught  with  great  difficulty,  and  even  yet 
different  views  are  held  by  chemists  as  to  the 
mechanism  of  dyeing. 

Although  unanimity  of  opinion  has  not  yet  been 
arrived  at,  there  are  certain  ideas,  which  will  be 
discussed  at  a  later  stage,  which  may  be  brought  into 
harmony  with  all  the  different  opinions,  and  so  it  is 
not  impossible,  unlikely  as  such  a  thing  may  at  first 
sight  appear,  to  arrive  at  a  theory  which  shall  apply 
to  all  cases  of  dyeing. 

80 


THEORIES  OF  DYEING  31 


The  Mechanical  Theory. 

The  earliest  idea  respecting  the  dyeing  process  was 
that  it  was  of  a  purely  mechanical  character.  Ex- 
planations of  the  process  on  these  lines  were  given 
about  the  middle  of  the  eighteenth  century  by  Hellot, 
Le  Pileur  d'Apligny,  and  others.  Hellot  stated  that 
the  heat  of  the  dyebath  causes  the  pores  of  the 
fibre  to  open  so  that  the  particles  of  colouring  matter 
can  enter  and  be  deposited ;  when  the  fibre  is  removed 
from  the  dyebath  the  pores  contract,  and  so  the  dye 
is  retained  in  position.  As  regards  the  various  sub- 
stances used  in  preparing  the  material  for  dyeing, 
these  were  also  considered  to  be  retained  in  the 
pores  of  the  fibre,  and  were  thought  to  coat  the 
particles  of  dye  with  a  kind  of  varnish. 

The  difference  in  the  behaviour  of  different  fibres 
towards  the  same  dyestuff  was  a  phenomenon  with 
which  the  early  dyers  were  thoroughly  familiar,  and 
this  was  also  explained  on  a  purely  mechanical  basis, 
Le  Pileur  d'Apligny  suggesting  that  it  was  to  be 
attributed  to  the  difference  in  size  of  the  pores  of 
the  various  fibres,  so  that  in  some  cases  the  particles 
of  dye  were  too  large  to  enter  the  pores,  or,  if  they 
entered,  the  contraction  on  cooling  was  insufficient 
to  permit  of  the  dye  being  retained. 

Although  the  majority  of  chemists  at  the  present 
day  are  of  the  opinion  that  the  dyeing  of  a  piece  of 
wool  or  cotton  is  not  to  be  accounted  for  in  such  a 
simple  manner  as  the  above  ideas  would  suggest, 
yet  there  are  some  who  still  adhere  to  the  mechanical 
theory  of  dyeing,  and  there  are  certain  cases  where 


32  THE  CHEMISTRY  OF  DYEING 

the  fibre  certainly  does  appear  to  act  in  a  mechanical 
manner.  Amongst  the  later  supporters  of  this  theory 
may  be  mentioned  Hwass,33  von  Perger,46  and  Spohn.55 

Stress  is  laid  on  the  fact  that  no  definite  compound 
of  a  fibre  and  a  dyestuff  has  ever  been  actually  shown 
to  be  produced  during  the  dyeing  process,  and  also  on 
the  retainment  of  many  of  its  original  properties  by 
a  colouring  matter  after  it  has  been  fixed  upon  the 
fibre.  The  rubbing  off  of  the  colour  which  is  fre- 
quently observed  with  dyed  goods,  and  the  possibility 
in  some  cases  of  separating  the  dye  from  the  fibre 
by  the  process  of  sublimation,  are  also  considered  to 
favour  the  mechanical  conception  of  the  character  of 
the  dyeing  process. 

A  property  common  to  all  dyestuffs  is,  that  if  a 
fibre  is  introduced  into  a  very  dilute  solution  of  the 
colouring  matter  practically  the  whole  of  it  is  taken 
up  and  is  firmly  retained,  even  when  the  dye  is  one 
for  which  the  fibre  in  question  may  naturally  have 
little  affinity.  This  phenomenon  is  attributed  to  the 
forces  of  capillarity  and  adhesion,  and  it  is  argued 
by  the  supporters  of  the  theory  that  similar  forces 
must  come  into  play  and  be  responsible  for  all  the 
results  which  follow  the  introduction  of  a  fibre  into 
a  more  concentrated  solution  of  a  colouring  matter, 
such  as  is  actually  employed  for  the  dyeing  of  a 
fabric. 

While  it  is  probable  that  the  forces  of  adhesion 
and  capillary  attraction  do  come  into  play  to  a  con- 
siderable extent,  it  is  scarcely  justifiable  to  conclude 
that  all  the  phenomena  of  dyeing  can  be  explained 
as  resulting  from  the  operation  of  those  forces. 


THEORIES  OF  DYEING  33 

Reference  may  also  be  made  to  the  views  of  Rosen- 
stiehl,50  who  pointed  out  that  solids,  under  the  influence 
of  pressure,  can  be  made  to  adhere  rigidly  to  one 
another.  He  considered  the  fixation  of  dyes  to  be 
brought  about  in  this  manner,  the  pressure  necessary 
to  make  the  dye  adhere  to  the  fibre  being  the  osmotic 
pressure  of  the  solution,  this  osmotic  pressure  being 
increased  by  the  addition  to  the  dyebath  of  the 
assistants,  such  as  acids  and  salts,  commonly  added 
to  the  bath. 

The  experiments  of  Dreaper  and  Wilson17  go  to 
show  that  to  regard  the  dyeing  process  as  a  purely 
mechanical  one  is  to  take  up  an  untenable  position. 
It  was  shown  that  when  Night  Blue  is  absorbed  by 
silk  at  a  temperature  of  15°  the  whole  of  the  colour- 
ing matter  can  be  subsequently  removed  from  the 
fabric  by  means  of  alcohol  or  by  treatment  with  a 
boiling  solution  of  soap;  if,  however,  dyeing  takes 
place  at  a  temperature  of  40°  and  upwards  a  portion 
of  the  dye  appears  to  be  taken  up  in  a  different 
manner,  and  cannot  be  removed  by  means  of  the 
above-mentioned  agents.  In  all  these  experiments 
the  total  quantity  of  the  dye  upon  the  fibre  was 
maintained  constant,  so  that  the  results  must  really 
be  due  to  a  difference  in  the  mode  of  existence  of  the 
colouring  matter  when  fixed  under  the  different 
conditions  of  temperature.  Similar  results  were 
obtained  in  the  case  of  a  number  of  other  colouring 
matters.  In  these  cases,  therefore,  it  appears  obvious 
that  something  in  addition  to  mere  mechanical  action 
is  necessary  to  explain  the  fixation  of  the  dye. 

There  is  one  sense,  however,  in  which  the 

C 


34  THE  CHEMISTRY  OF  DYEING 

mechanical  theory  may  be  accepted,  and  that  is  in 
connection  with  the  fixation  of  mordant  colours,  the 
so-called  mineral  colouring  matters  and  the  ingrain 
colours  (that  is,  the  azo  dyes  produced  on  the  fibre). 
Spohn  (loc.  cit.)  examined  with  the  aid  of  the  micro- 
scope cotton  which  had  been  dyed  with  lead  chromate, 
and  noticed  the  crystals  of  the  colouring  matter 
adhering  to  the  colourless  fibre.  In  such  a  case  the 
colour  is  simply  deposited  in  the  pores  of  the  fibre 
as  it  is  formed  by  the  interaction  of  the  two  reacting 
substances,  and  under  such  conditions  the  part  played 
by  the  fibre  is  the  purely  mechanical  one  of  a 
pigment  carrier. 

This  view  of  adjective  or  mordant  dyeing  was 
supported  by  Weber,59  who  pointed  out  the  difference 
in  behaviour  /shown  by  Night  Blue  when  applied 
to  cotton  in  the  absence  of  a  mordant  and  in  the 
presence  of  tannic  acid.  Under  the  former  conditions 
the  colouring  matter  retained  its  original  properties 
and  at  once  reacted  with  Naphthol  Yellow,  but  when 
fixed  in  the  presence  of  tannic  acid  no  reaction 
ensued  on  subsequently  treating  the  dyed  cotton  with 
the  yellow  dye. 

The  Chemical  Theory. 

Some  little  time  after  the  period  of  Hellot,  etc., 
Bergmann  advanced  the  view  that  the  process  of 
dyeing  is  to  be  regarded  as  being  of  a  purely  chemical 
character,  an  interaction  taking  place  between  the 
fibre  and  the  dye.  Berthollet  and  others  supported 
this  view,  which  has  persisted  to  the  present  day. 


THEORIES  OF  DYEING  35 

There  is  much  to  be  said  in  favour  of  such  an 
explanation  of  the  dyeing  process.  With  regard  to 
the  dyestuffs  themselves,  it  has  already  been  pointed 
out  that  the  presence  of  groups  of  an  acid  or  basic 
character  is  necessary  before  a  substance  can  behave 
as  a  dye. 

When  we  turn  to  the  fibres,  we  find  that  the 
animal  fibres  possess  an  amphoteric  character  by 
virtue  of  which  it  would  be  possible  for  them  to 
unite  either  with  a  base  or  an  acid  with  the  formation 
of  a  salt.  This  is  in  fact  the  way  in  which  the 
advocates  of  the  chemical  theory  of  dyeing  explain 
the  substantive  dyeing  of  wool  and  silk  with  the 
basic  and  acid  dyes. 

As  regards  cotton,  the  fact  that  it  is  practically 
devoid  of  acid  or  basic  properties  makes  it  an  im- 
possibility for  any  salt  to  be  formed  by  the  interaction 
of  fibre  and  dyestuff,  and  the  comparative  inertness 
of  cotton  towards  colouring  matters  is  thus  accounted 
for. 

When  we  consider  jute,  we  find  that  the  fibre  can 
be  dyed  directly  with  the  dyes  of  the  basic  class, 
whereas  such  dyes  can  only  be  fixed  upon  cotton 
after  the  fibre  has  been  mordanted  with  tannic  acid 
or  some  other  acid  mordant.  The  difference  in  com- 
position of  cotton  and  jute  makes  it  possible  to  easily 
explain  this  difference,  for  as  has  been  already  stated, 
jute  is  composed  of  ligno-cellulose,  which  is  not  un- 
like, in  properties,  cellulose  which  has  been  mordanted 
with  tannic  acid.  Here  again,  therefore,  the  difference 
in  behaviour  towards  colouring  matters  can  be  easily 
explained  by  taking  into  account  the  chemical  nature 


36  THE  CHEMISTRY  OF  DYEING 

of  the  fibre.  Certain  of  the  objections  against  the 
theory  have  been  mentioned  in  dealing  with  the 
mechanical  theory. 

A  large  amount  of  experimental  work  has  been 
carried  on  during  the  last  twenty-five  years  in  support 
of  the  ideas  embodied  in  the  foregoing  general  state- 
ment. Knecht 34  showed  that  when  wool  wras  boiled 
with  a  moderately  concentrated  solution  of  sulphuric 
acid  it  gradually  dissolved  and  gave  rise  to  a  light 
browTn  solution ;  if  solutions  of  dyestuffs  were  added 
to  the  acid  solution  richly  coloured  precipitates  were 
formed.  By  the  careful  neutralisation  of  the  acid 
solution  a  substance  was  precipitated  which,  on 
drying,  yielded  an  amorphous  brown  powder  which 
was  only  sparingly  soluble  in  acids,  but  dissolved 
readily  in  solutions  of  alkalis;  when  the  alkaline 
solution  was  mixed  with  solutions  of  colouring  matters 
and  an  acid  then  added,  precipitates  were  formed 
similar  to  those  which  were  obtained  from  the 
original  acid  solution.  From  these  results  Knecht 
drew  the  conclusion  that  the  principal  reason  why 
sulphuric  acid  is  added  to  the  dyebath  in  the  dyeing 
of  wool  is  to  act  upon  the  fibre  so  as  to  produce 
a  compound  having  properties  similar  to  those  possessed 
by  the  substance  isolated;  the  compound  so  formed 
would  then  combine  with  the  dye  to  form  an 
insoluble  product. 

Reference  has  been  made  in  an  earlier  chapter  to 
the  experiments  made  by  Knecht  and  Appleyard37 
with  lanuginic  acid  which  they  prepared  from  wool. 
When  solutions  of  basic  colouring  matters  were 
added  to  a  solution  of  lanuginic  acid,  precipitates 


THEORIES  OF  DYEING  37 

were  at  once  produced,  and  it  was  considered  that 
this  kind  of  change  probably  took  place  during  the 
dyeing  of  wool  with  dyes  of  the  basic  class,  a  certain 
quantity  of  the  fibre  first  undergoing  decomposition 
with  the  formation  of  lanuginic  acid,  which  would 
then  react  with  the  colouring  matter. 

Amongst  other  results  obtained  by  Knecht  and 
Appleyard  (loc.  cit.\  some  of  a  quantitative  character 
may  be  referred  to.  When  wool  was  dyed  with 
certain  acid  dyes,  it  was  found  that  if  the  quantity 
of  one  colouring  matter,  Picric  Acid,  absorbed  by 
a  given  quantity  of  wool,  was  taken  as  the  unit, 
then  the  amounts  of  two  other  dyes,  Naphthol  Yellow 
S  and  Tartrazine,  absorbed  by  the  same  weight  of 
wool,  corresponded  to  one  and  to  three-quarters  of 
a  molecule  respectively ;  and  the  obtaining  of  these 
quantitative  results  was  thought  to  be  a  strong  piece 
of  evidence  in  support  of  the  chemical  theory. 
Further  quantitative  results  were  given  by  Knecht36 
in  a  later  paper. 

Gelmo  and  Suida2456  also  came  to  the  conclusion 
that  in  the  substantive  dyeing  of  the  animal  fibres 
it  is  necessary  for  the  fibre  to  first  undergo  a  more 
or  less  profound  hydrolysis,  as  a  result  of  which 
products  are  formed  which  contain  active  salt-forming 
groups.  This  hydrolysis  is  greater  in  the  case  of 
wool  than  with  silk,  and  is  promoted  by  the  presence 
of  acid,  and  is  necessary,  according  to  the  authors 
in  question,  for  the  satisfactory  dyeing  of  the  fibres. 
As  regards  the  fixation  of  the  dye,  Gelmo  and  Suida 
considered  that  basic  dyes  are  fixed  by  salt  formation 
with  the  acid  groups  of  the  fibre  substance,  and  acicj 


38  THE  CHEMISTRY  OF  DYEING 

dyes  by  means  of  guanidyl  or  imidazole  groups  of 
the  fibre. 

It  will  be  noticed  from  the  preceding  statements 
that  by  some  supporters  of  the  chemical  theory,  at 
any  rate,  the  salt  formation  which  is  supposed  to 
take  place  is  not  one  involving  particular  groups  of 
the  fibre  itself,  but  groups  present  in  some  product 
formed  by  the  hydrolysis  of  the  fibre.  If  this  is 
really  the  nature  of  the  action,  then,  according  to 
Witt,60  the  dyeing  of  wool  and  silk  with  the  acid 
and  basic  colouring  matters  should  not  be  regarded 
as  examples  of  substantive  dyeing,  but  as  instances 
of  adjective  dyeing,  the  products  of  the  hydrolysis 
of  the  fibre  acting  as  the  mordant. 

Other  chemists,  however,  appear  to  regard  the  action 
as  one  in  which  the  salt-forming  groups  of  the  fibres 
themselves  are  involved,  and  if  such  were  the  nature 
of  the  process  then  the  operation  would  undoubtedly 
be  one  of  substantive  dyeing.  Amongst  those  who 
may  be  said  to  have  favoured  this  view  was  Weber,59 
who  carried  out  some  experiments  of  an  interesting 
character.  He  argued  that  if  the  process  of  the 
substantive  dyeing  of  wool  and  silk  is  one  in  which 
the  fibre  may  be  regarded  as  playing  the  part  of 
an  amphoteric  compound,  then  even  although  an  acid 
dye  has  been  fixed  upon  the  fibre  it  should  still  be 
possible  to  fix  a  basic  one  upon  the  same  material, 
seeing  that  the  acid  group  in  the  wool  by  means 
of  which  the  basic  dye  is  fixed  would  still  be  free. 
This  was  actually  found  to  be  possible  in  practice, 
a  skein  of  wool  being  dyed  with  a  large  excess  of 
Scarlet  B,  and  then,  after  thorough  washing,  intrq- 


THEORIES  OF  DYEING  39 

duced  along  with  a  white  skein  of  equal  weight 
into  a  solution  of  Magenta.  It  was  found  that  the 
same  weight  of  Magenta  was  absorbed  by  both  skeins, 
thus  showing  the  acid  colouring  matter  to  have  no 
influence  on  the  quantity  of  basic  dye  taken  up. 
Of  course  it  might  be  argued  against  this  experiment 
that  union  had  taken  place  between  the  two  dyestuffs, 
but  any  such  objection  was  confuted  by  the  behaviour 
of  the  dyed  material  when  treated  with  alcohol. 
The  substances  formed  by  the  union  of  acid  and 
basic  dyes  are  readily  soluble  in  that  liquid,  but 
it  was  found  that  only  a  small  amount  of  the 
Magenta  and  none  of  the  Scarlet  was  removed  from 
the  material  by  the  solvent. 

A  number  of  experiments  were  carried  out  by 
Vignon,57  which  appeared  to  support  the  chemical 
theory  of  dyeing.  Cotton,  it  is  well  known,  has 
practically  no  affinity  for  the  acid  dyes,  and  this 
is  attributed  to  the  absence  of  groups  of  a  basic 
character  in  the  fibre.  Vignon  showed,  however, 
that  when  cotton  is  heated  in  sealed  tubes  with  an 
aqueous  solution  of  ammonia  or  with  the  compound 
of  ammonia  with  calcium  chloride;  its  properties 
undergo  considerable  modification.  After  the  material 
has  been  thoroughly  washed  with  water  and  acids 
it  is  found  to  contain  nitrogen,  the  amount  of  which 
may  reach  3  per  cent.,  and  when  a  piece  of  the 
modified  cotton  is  placed  in  a  solution  of  an  acid 
dye  in  company  with  a  piece  of  untreated  cotton, 
a  considerable  amount  of  the  colouring  matter  is 
taken  up  by  the  modified  fibre,  while  the  ordinary 
Cptton  is  scarcely  stained.  On  the  other  hand,  oxy- 


40  THE  CHEMISTRY  OF  DYEING 

cellulose,  which  has  acid  properties,  has  a  greater 
attraction  than  ordinary  cotton  for  basic  dyes. 

Vignon  also  measured  the  amount  of  heat  which 
is  generated  when  different  fibres  are  immersed  in 
solutions  of  acids  and  alkalis.  The  heat  effect  pro- 
duced by  cotton  under  such  conditions  is  very  small 
when  compared  with  the  effect  produced  with  wool 
or  silk,  and  this  was  considered  as  pointing  to  a 
certain  amount  of  chemical  action  taking  place  with 
the  animal  fibres,  as  against  practically  no  action 
with  the  cotton.  This  conclusion  was  strengthened 

e 

by  the  observation  that  when  the  ammoniated  cotton 
already  referred  to  was  immersed  in  sulphuric  acid 
a  considerable  amount  of  heat  was  generated. 

Physico-Chemical  Objections  to  the  Chemical 
Theory. — With  the  rise  and  development  of  physical 
chemistry,  some  of  the  arguments  formerly  brought 
forward  in  favour  of  this  explanation  of  the  dyeing 
process  have  been  shown  to  have  little  bearing  on 
the  subject.  It  was,  for  example,  noticed  by 
Knecht35  that  when  wool  is  dyed  by  means  of  a 
basic  dye  such  as  Magenta,  the  whole  of  the  hydro- 
chloric acid  originally  present  in  the  colouring  matter 
remains  in  the  dyebath,  the  colour  base  alone  being 
taken  up  by  the  fibre. 

At  first  sight  this  appears  to  be  conclusive  evidence 
in  favour  of  the  chemical  theory,  but  it  was  found 
by  von  Georgievics 25  that  if  the  operation  is  carried 
out  at  a  temperature  below  the  boiling-point  of  water, 
the  whole  of  the  halogen  of  the  dyestuff  is  not  present 
in  the  bath  at  the  conclusion  of  the  dyeing  process, 
but  a  portion  is  taken  up  by  the  fibre,  Moreover, 


CRITICISM  OF  CHEMICAL  THEORY          41 

if  glass  beads  or  pieces  of  unglazed  earthenware  are 
introduced  into  a  solution  of  Magenta  at  the  ordinary 
temperature,  a  certain  quantity  of  colour  is  acquired 
by  such  articles,  and  the  chlorine,  at  the  end  of  the 
process,  is  present  quantitatively  in  the  bath;  in 
cases  such  as  these  there  is  no  possibility  of  chemical 
action  taking  place  between  the  dyed  object  and  the 
colouring  matter,  and  some  other  explanation  of  the 
result  must  be  sought  for  in  place  of  that  given  by 
Knecht. 

Such  an  explanation  was  suggested  by  Zacharias,61 
who  pointed  out  that  the  dyes  have  the  character  of 
salts,  and  may  therefore  be  more  or  less  hydrolysed 
in  aqueous  solution  into  free  colour  base  and  free 
acid.  As  in  all  such  cases,  there  will  be  a  certain 
degree  of  hydrolysis  depending  on  the  concentration 
of  the  solution  and  on  the  temperature.  As  long  as 
nothing  happens  to  disturb  the  equilibrium,  no  further 
change  will  take  place.  The  introduction  of  a  piece 
of  wool  into  the  solution  has,  however,  a  disturbing 
effect,  since  it  leads  to  the  removal  of  one  of  the 
products  of  hydrolysis,  the  colour  base,  and  in 
accordance  with  the  laws  of  chemical  equilibrium 
a  further  amount  of  the  salt  must  be  hydrolysed  to 
restore  the  condition  of  equilibrium  in  the  solution ; 
it  is  at  once  evident  that  this  process,  if  continued, 
will  lead  eventually  to  the  complete  decomposition 
of  the  dyestuff.  The  effect  of  temperature  noticed 
by  von  Georgievics  can  also  be  explained  quite  easily 
on  the  same  basis,  for  in  the  majority  of  cases  the 
degree  of  hydrolysis  of  a  salt  increases  with  the 
temperature,  so  that  at  temperatures  below  the  boiling- 


42  THE  CHEMISTRY  OF  DYEING 

point  the  process  of  hydrolysis  might  not  be  complete, 
the  products  of  hydrolysis  being  a  basic  salt  and  some 
free  acid ;  the  basic  salt  would  be  absorbed  by  the 
fibre,  which  would  therefore  contain  a  certain  amount 
of  halogen. 

Coloured  and  Colourless  Modifications  of  Dye- 
bases. — Emphasis  was  also  laid  by  the  advocates  of 
the  chemical  theory  on  the  fact  that  when  wool  is 
introduced  into  water  in  which  the  colourless  base 
of  Magenta  is  suspended,  the  fibre  acquires  the 
magenta  colour  characteristic  of  solutions  of  the  salts. 
It  was  pointed  out  by  von  Georgievics  (loc.  cit.)  that 
it  did  not  necessarily  follow  from  this  experiment 
that  salt  formation  had  taken  place  between  the 
fibre  and  the  colour  base;  the  coloration  of  the 
fibre  might  be  due  simply  to  a  change  in  the  con- 
stitution of  the  colour  base.  The  ordinary  colour 
base  has  the  carbinol  form  and  is  colourless,  but  von 
Georgievics  obtained  a  second  variety  in  which'  the 
substance  has  a  quinonoid  structure,  and  this  modifica- 
tion has  a  magenta  colour,  and  is  probably  the  base 
to  which  the  dye  salts  really  correspond.  The 
existence  of  this  second  form  of  the  base  was  also 
indicated  by  electrical  conductivity  measurements 
made  by  Hantzsch  and  Osswald,31  who  were  of  the 
opinion  that  the  bases  of  other  dyes  of  the  triphenyl- 
methane  series  existed  also  in  two  different  modi- 
fications. 

/C6H4NH2  /C6H4NH2 

HO— C— C6H4N  H2  C— C6H4N  H2 

\C6H4NH2  ^C6H4  =  NH2OH 

Cojourjesg  form  (para-rosaniline).  Coloured  fprm  (para-rosanjline), 


CRITICISM  OF  CHEMICAL  THEORY          43 

In  view  of  the  existence  of  this  second  modification 
of  the  colour  base,  it  is  obvious  that  the  production 
of  colour  when  a  colourless  form  of  a  base  is  taken 
up  by  a  fibre  does  not  necessarily  indicate  the 
formation  of  a  salt. 

Other  Objections  to  the  Chemical  Theory. — One 
of  the  difficulties  in  the  way  of  the  chemical  theory  is 
that  of  explaining  the  behaviour  of  fibres  towards 
the  colouring  matters  of  the  direct  cotton  group. 
Both  cotton  and  the  animal  fibres  are  dyed  directly 
by  these  colouring  matters,  a  fact  which  cannot  be 
explained  by  any  theory  which  involves  the  chemical 
nature  of  the  fibre. 

The  recent  experiments  of  Dreaper  and  Wilson,18 
who  showed  that  acid  dyes  can  be  applied  to  silk 
from  an  alkaline  bath,  are  also  scarcely  favourable 
to  the  chemical  theory  of  dyeing. 

The  behaviour  of  dyed  articles  towards  certain 
solvents  was  also  considered  by  Witt60  to  be  difficult 
to  explain  by  means  of  the  chemical  theory.  Silk, 
for  example,  which  has  been  dyed  with  Magenta 
requires  to  be  treated  with  a  moderately  concentrated 
soap  solution  before  it  gives  up  any  of  the  colouring 
matter  which  has  been  fixed  upon  it,  and  one  might 
therefore  argue  that  the  Magenta  and  the  silk  form 
a  compound  of  considerable  stability.  When,  how- 
ever, the  dyed  silk  is  placed  in  absolute  alcohol  the 
dye  is  removed  almost  instantaneously  from  the 
fibre.  In  this  latter  case  no  chemical  operation  has 
been  involved,  the  relations  between  the  dye  and 
the  alcohol  being  simply  those  of  solute  and  solvent. 
On  the  addition  of  water  to  the  alcoholic  solution, 


44  THE  CHEMISTRY  OF  DYEING 

a  certain  quantity  of  the  colouring  matter,  the  actual 
amount  depending  on  the  degree  of  dilution  of  the 
alcohol,  returns  to  the  silk. 

Problem  of  the  Unexhausted  Dyebath. — A  matter 
of  a  similar  nature  to  the  foregoing  is  the  impossibility 
often  experienced  in  practice  of  completely  exhausting 
the  dyebath.  If,  after  the  material  has  been  dyed 
as  fully  as  possible  in  a  bath  of  this  type,  it  is 
removed  from  the  bath  and  a  second  portion  intro- 
duced, some  of  the  remaining  colouring  matter  will 
be  taken  up  by  the  new  material,  but  not  the  whole 
of  it,  and  this  process  can  be  repeated  a  number  of 
times  without  completely  exhausting  the  dyebath. 

It  is  difficult  to  explain  results  such  as  these  by 
means  of  the  chemical  theory,  for  if  the  process  of 
dyeing  simply  consists  in  the  formation  of  a  com- 
pound between  the  fibre  and  the  colouring  matter,  how 
is  it  that  the  introduction  of  a  further  quantity  of 
the  former  does  not  result  in  the  complete  removal 
of  the  latter  from  the  dyebath  ?  Of  course,  behaviour 
of  this  kind  is  not  incompatible  with  the  formation 
of  a  compound  of  fibre  and  dye ;  it  might  simply  be 
a  case  of  the  establishment  of  equilibrium  in  a 
heterogeneous  system  made  up  of  the  undyed  fibre, 
the  dyed  fibre,  and  the  dyestuff  in  solution.  But  in 
such  a  system,  where  two  of  the  substances  are 
practically  insoluble  and  the  third,  the  dye,  soluble, 
the  condition  of  equilibrium,  as  arrived  at  from 
the  application  of  the  Law  of  Mass  Action,  is  that 
for  each  temperature  there  will  be  a  certain  con- 
centration of  dye  solution  with  which  the  dyed 
fibre  can  be  in  equilibrium.  If  the  concentration  of 


CRITICISM  OF  CHEMICAL  THEORY          45 

the  solution  exceeds  this  equilibrium  value,  then  dye 
will  be  taken  up  by  the  fibre  until  the  concentration  is 
reduced  to  the  necessary  degree,  after  which  no 
further  dyeing  will  take  place ;  if,  on  the  other  hand, 
a  piece  of  undyed  fibre  should  be  placed  in  a  solution 
of  a  dye,  the  concentration  of  which  is  lower  than  the 
equilibrium  value,  no  dyeing  will  ensue. 

The  experiments  of  Walker  arid  Appleyard 58  showed 
that  these  requirements  were  fulfilled  by  diphenyl- 
amine  when  dyed  with  picric  acid,  but  in  no  known 
case  of  ordinary  dyeing  are  the  theoretical  require- 
ments satisfied,  for  no  matter  how  dilute  a  solution  of 
a  colouring  matter  may  be,  a  certain  amount  of  the 
dye  will  be  taken  up  from  it  on  the  introduction  of 
a  fibre.  The  question  of  the  unexhausted  dyebath 
remains  therefore  a  difficult  one  to  explain  by  means 
of  the  chemical  theory. 

Experiments  with  a  Liquid  as  an  Artificial 
Substitute  for  a  Fibre. — Prud'homme48  thought  he 
might  obtain  information  ds  to  the  part  played  by 
acid  and  basic  groups  of  the  fibre  in  fixing  dyestuffs 
by  carrying  out  experiments  using  an  artificial 
substitute  for  a  fibre  in  the  form  of  a  liquid.  He 
chose  a  neutral  substance  which  was  immiscible  with 
water,  such  as  benzene,  chloroform,  or  amyl  alcohol, 
and  in  this  liquid  dissolved  an  organic  acid  such  as 
salicylic  acid,  a  weak  base,  preferably  an  imide 
like  acetanilide,  or  a  mixture  of  acid  and  base; 
these  solutions  corresponded  to  the  fibre.  Small 
quantities  of  the  bases  of  various  basic  dyes  were 
dissolved  in  a  dilute  solution  of  sodium  hydroxide, 
and  two  equal  quantities  of  the  different  solutions 


46  THE  CHEMISTRY  OF  DYEING 

were  then  measured  and  mixed,  the  one  with  amyl 
alcohol  and  the  other  with  the  same  substance 
containing  10  per  cent,  of  salicylic  acid.  The 
mixtures  were  rapidly  shaken,  when  it  was  invari- 
ably found  that  the  neutral  solution  had  the  natural 
colour  of  the  base,  while  the  other  solution  became 
coloured  like  the  salts  of  the  base. 

As  silk  when  dyed  with  the  same  colouring  matters 
acquires  the  same  colour  as  the  acid  amyl  alcohol,  it 
was  considered  that  these  results  showed  that  the 
essential  feature  in  the  dyeing  of  silk  with  basic 
colouring  matters  is  the  union  of  the  base  with  the 
acid  group  of  the  fibre.  In  the  same  manner  it  was 
-  found  that  when  acidified  solutions  of  acid  colouring 
matters  were  employed,  the  colour  which  resulted 
when  such  a  solution  was  shaken  with  a  solution  of 
acetanilide  in  amyl  alcohol  was  identical  with  that 
acquired  by  wool  and  silk  when  treated  with  the 
same  dye,  and  it  was  accordingly  considered  as  proved 
that  the  essential  feature  of  the  dyeing  of  animal 
fibres  with  acid  dyes  was  salt  formation  between 
the  acid  of  the  dye  and  the  basic  group  of  the  fibre. 

Exception  was  taken  by  Gillet 30  to  the  use  of  sali- 
cylic acid  and  acetanilide  in  Prud'homme's  experiments, 
on  the  ground  that  those  substances  are  appreciably 
soluble  in  water,  so  that  the  solution  in  amyl  alcohol 
can  scarcely  be  compared  with  wool  or  silk,  the  acid 
and  basic  groups  of  which  are  not  soluble  in  water. 
Gillet  employed  a  5  per  cent,  solution  of  /3-naphthol 
in  amyl  alcohol  in  place  of  the  solution  of  salicylic 
acid,  and  obtained  results  with  the  aqueous  solutions 
of  colour  bases  which  confirmed  those  arrived  at  by 


ABSORPTION  OF  DYES  BY  LIQUIDS         47 

Prud'homme.  The  results  with  acid  dyes,  using  a 
solution  of  /3-naphthylamine,  were  not  very  conclusive. 
Gillet  doubted  whether  the  acid  dyes  were  fixed  by 
virtue  of  their  acid  groups,  but  thought  it  more  likely 
that  it  was  through  the  feebly  basic  groups  often 
present  in  acid  dyes  that  union  with  the  fibre  took 
place. 

Sisley53  criticised  the  results  obtained  by  Prud'- 
homme, Gillet,  etc.,  and  stated  that  amyl  alcohol 
itself  becomes  coloured  when  warmed  with  alkaline 
solutions  of  colour  bases,  the  intensity  of  the  colour 
depending  on  the  alkalinity  of  the  solution.  These 
results  obtained  by  Sisley  were  attributed  by  his 
opponents 29  49  to  the  presence  of  small  amounts  of 
acid  impurities  in  the  amyl  alcohol  used  by  him, 
but  he  maintained  that  such  was  not  the  case,  and 
that  even  after  thorough  purification  of  the  alcohol, 
the  colour  was  obtained  on  warming  with  the  solution 
of  the  colour  base.  In  view  of  this  observation, 
no  great  support  for  the  chemical  theory  can  be 
found  in  the  results  arrived  at  by  Prud'homme  and 
others.  Sisley  (loc.  cit.)  was  of  the  opinion  that 
these  results  were  better  explained  by  a  change  in 
the  structure  of  the  colour  base,  such  as  had  been 
proved  to  happen  with  Magenta,  rather  than  by  salt 
formation  with  the  dissolved  acid. 

Nature  of  the  Reactive  Group  in  the  Fibre. — It 
will  have  been  noticed  that  several  ideas  have  been 
expressed  regarding  the  kind  of  groups  in  the  fibre 
with  which  acid  dyes  in  particular  are  supposed  to 
combine  to  form  a  salt-like  product.  Experiments 
conducted  by  Bentz  and  Farrell4  appear  to  show 


48  THE  CHEMISTRY  OF  DYEING 

that  the  union  is  not  at  any  rate  with  the  aromatic 
primary  amino  group  contained  in  the  fibre.  As 
was  mentioned  in  connection  with  the  fibres,  both 
wool  and  silk  contain  a  group  of  this  description, 
which  can  be  diazotised  in  the  usual  manner,  and 
the  diazo  compound  then  coupled  with  phenols.  If 
the  diazo  compound  is  boiled  with  water,  alcohol,  or 
a  solution  of  cuprous  chloride,  the  nitrogen  originally 
present  as  the  aromatic  amino  group  is  removed. 
Bentz  and  Farrell  showed  that  wool  and  silk  which 
had  been  treated  in  this  manner  behaved  towards 
acid  dyes  in  just  the  same  manner  as  the  normal 
fibres,  the  dyebaths  being  equally  exhausted  and  the 
dyeings  equally  fast. 

Binz  and  Schroeter  9  suggested  that  a  union  between 
fibre  and  dye  might  take  place  altogether  different 
from  the  salt  formation  which  most  advocates  of  the 
chemical  theory  of  dyeing  regard  as  the  nature  of 
the  action.  They  pointed  out  that  if  the  chemical 
theory  in  its  ordinary  form  is  true,  the  affinity  of 
dyes  for  the  fibre  would  be  a  function  of  their  salt- 
forming  power.  This  does  not  agree  with  many 
observations,  for  it  has  been  noticed  that  the  introduc- 
tion into  a  chromogen  of  a  sulphonic  or  carboxyl 
group  does  not  confer  any  marked  dyeing  power 
on  the  substance,  whereas  the  introduction  of  the 
amino  and  phenolic  groups,  whose  salt -forming  power 
is  much  smaller,  leads  to  the  production  of  some  of 
our  most  powerful  dyes.  A  number  of  experiments 
were  made  with  derivatives  of  azobenzene,  and  the 
conclusion  was  drawn  that  when  amino,  sulphonic, 
and  other  groups  were  introduced  into  the  meta 


REACTIVE  GROUPS  IN  FIBRES  49 

position  with  regard  to  the  azo  group,  the  substance 
formed  had  a  certain  degree  of  dyeing  power,  the 
mechanism  of  dyeing  appearing  to  be  of  the  nature 
of  salt  formation.  When,  however,  the  substituting 
group  entered  in  the  para  position  to  the  azo  group, 
the  substances  formed  were  powerful  dyes,  and  the 
process  of  dyeing  appeared  to  be  different  in  character, 
since  it  was  not  reversible  according  as  acid  or  alkali 
was  present.  Many  of  the  fast  wool  and  silk  dyes 
can  be  represented  as  having  a  quinonoid  structure, 
and  Binz  and  Schroeter  considered  it  probable  that 
they  are  fixed  by  a  kind  of  ring  condensation  between 
the  fibre  and  the  dye,  salt  formation  only  coming 
in  as  a  secondary  factor. 

The  Solution  and  Adsorption  Theories. 

It  has  already  been  shown  that  Witt 60  considered 
the  chemical  theory  in  many  respects  unsatisfactory, 
and,  in  1890,  he  brought  forward  another  explanation 
of  the  dyeing  process  which  he  considered  to  be  more 
in  harmony  with  the  observed  facts.  The  new  theory, 
according  to  which  the  dye  was  supposed  to  exist 
in  the  fibre  in  a  state  of  solution,  may  be  regarded 
as  occupying  a  position  intermediate  between  the 
two  older  theories.  In  support  of  his  contention, 
•Witt  pointed  out  that  in  the  case  of  many  basic  dyes 
of  the  triphenylmethane  series,  such  as  Methyl  Violet 
and  Magenta,  the  colour  acquired  by  wool  or  silk 
when  immersed  in  a  solution  of  the  colouring  matter 
is  not  that  of  the  dry  solid  but  that  of  the  solution ; 
the  bronzy  green  colour  characteristic  of  the  solid 

D 


50  THE  CHEMISTRY  OF  DYEING 

dyes  is  missing  from  the  dyed  fabric.  If  shellac 
varnish  is  dissolved  in  alcohol,  and  some  Magenta 
is  then  added,  the  resultant  liquid  has  a  red  colour 
as  long  as  any  alcohol  remains;  when,  however,  the 
solvent  has  all  evaporated,  the  residue  has  the  bronzy 
colour  characteristic  of  the  solid  dyestuff.  The  in- 
ference is  that  the  colour  is  entirely  due  to  the 
existence  of  the  dye  in  the  state  of  solution,  Magenta 
being  soluble  in  alcohol  but  insoluble  in  shellac. 
Then,  again,  Rhodamine,  when  dissolved  in  alcohol, 
gives  rise  to  a  fluorescent  solution.  The  solid  dye- 
stuff  shows  no  fluorescence,  but  silk  which  has  been 
dyed  with  Rhodamine  does  exhibit  this  property,  and 
Witt  therefore  concluded  that  the  Rhodamine  fixed  on 
the  silk  must  be  present  in  a  state  of  solution. 

As  to  why  some  fibres  can  be  dyed  directly  with 
certain  colouring  matters,  while  in  other  cases  the 
agency  of  a  mordant  is  necessary  to  effect  dyeing, 
this  was  explained  by  Witt  by  saying  that  in  those 
cases  where  a  mordant  is  required  the  solubility 
of  the  dyestuff  in  the  fibre  substance  is  too  small 
to  permit  of  effective  dyeing.  It  is  not  that  the 
dye  is  actually  insoluble  in  the  fibre,  but  the 
solubility  is  of  such  small  dimensions  when  com- 
pared with  the  solubility  of  the  colouring  matter  in 
water  that  practically  no  colouring  matter  is  taken 
up  by  the  fibre.  It  will  be  seen,  therefore,  that 
according  to  the  solution  theory  of  the  process,  the 
substantive  dyeing  of  a  fibre  is  akin  to  the  extraction 
of  a  substance  from  an  aqueous  solution  by  means 
of  an  immiscible  solvent  in  which  the  solubility  is 
greater  than  in  water.  An  explanation  of  this  kind 


SOLUTION  THEORY  OF  DYEING  51 

certainly  provides  a  reason  for  the  non-exhaustion 
of  the  dyebath,  which  was  one  of  the  difficulties  of 
the  chemical  theory. 

Witt  only  considered  the  chemical  character  of  the 
fibre  to  be  of  importance  in  so  far  as  it  affected  the 
solvent  power  of  the  substance.  Silk  was  considered 
to  be  dyed  more  readily  than  other  fibres  because 
the  solubility  of  dyes  in  fibroin  is  greater  than  in 
keratin  or  in  cellulose ;  the  solubility  of  many  dyes 
in  cellulose  is  so  small  that  sodium  sulphate  or  some 
other  salt  is  added  in  order  to  diminish  the  solubility 
in  the  liquor  of  the  dyebath. 

As  regards  adjective  dyeing,  Witt  was  of  the 
opinion  that  the  question  of  solution  still  had  to 
be  considered.  In  such  cases  it  was,  however,  the 
mordant  which  was  dissolved  by  the  fibre,  and  the 
solution  so  obtained  then  reacted  with  the  colouring 
matter  to  form  an  insoluble  compound. 

There  are  certain  cases  where  the  dyed  material 
has  a  colour  different  from  that  belonging  to  the 
aqueous  solution,  and  Witt  anticipated  that  this 
might  be  brought  forward  as  an  argument  against 
his  theory.  He  pointed  out,  however,  that  phenomena 
of  this  kind  are  by  no  means  unknown  in  cases 
of  undoubted  solution,  as  for  instance  with  iodine, 
the  aqueous  solution  of  which  is  brown  in  colour, 
while  the  solution  in  chloroform  has  a  violet  colour. 
The  explanation  of  these  differences  of  colour  is  that 
the  iodine  exists  in  the  two  solutions  in  different 
conditions,  a  feeble  union  between  solvent  and  solute 
probably  taking  place  in  those  liquids  from  which 
brown  solutions  are  obtained;  the  differences  ob- 


52  THE  CHEMISTRY  OF  DYEING 

served  with  dyestuffs  might  be  due  to  a  similar 
cause. 

Objections  to  Witt's  Solution  Theory. — It  must 
be  admitted  that  the  position  taken  up  by  Witt, 
and  the  arguments  advanced  in  support  of  his  ideas, 
strike  one  at  first  in  quite  a  favourable  manner,  but 
on  further  consideration  certain  objections  suggest 
themselves.  It  was  pointed  out  by  von  Georgievics 25 
that  the  phenomenon  of  fluorescence  is  shown  by  a 
number  of  solid  substances,  and  is  not  confined  to 
solutions.  Moreover,  although  silk  which  has  been 
dyed  with  Fluorescein  is  fluorescent,  yet  wool  which 
has  been  dyed  with  the  same  substance  shows  no 
fluorescence,  and  we  should  therefore,  according  to 
the  solid  solution  theory,  have  to  conclude  that  the 
Fluorescein  is  dissolved  by  the  silk  and  not  by  the 
wool,  although  both  fibres  are  in  the  dyed  state. 

The  arguments  of  Witt,  based  on  the  colour  of 
Magenta,  were  also  shown  to  be  untenable,  for  von 
Georgievics  demonstrated  that  when  solid  Magenta 
is  rubbed  between  two  glass  plates  it  loses  its  bronze 
colour  and  becomes  red.  Bronziness,  on  the  other 
hand,  is  sometimes  observed  on  dyed  goods,  as  when 
wool  is  dyed  with  a  concentrated  solution  of  Magenta. 
But  one  of  the  most  apparently  destructive  pieces 
of  criticism  brought  by  von  Georgievics2527  against 
the  solid  solution  theory  was  that  the  dyeing  process 
is  not  reversible.  If  a  dyed  fibre  is  simply  a  solution, 
then  one  would  expect  that  when  such  a  fibre  was 
placed  in  fresh  water  it  would  give  up  a  portion 
of  the  colouring  matter  which  it  had  absorbed,  no 
matter  what  the  nature  of  the  dyestuff  might  be ; 


CRITICISM  OF  SOLUTION  THEORY          53 

this,  however,  is  by  no  means  the  general  behaviour 
of  dyed  goods,  for  many  dyes  are  quite  fast  to 
washing. 

Another  anomaly  arises  from  the  fact  that  wool 
is  more  readily  dyed  from  a  boiling  than  from  a 
cold  solution  of  a  dye,  and  takes  up  a  larger  quantity 
of  colouring  matter  under  the  former  than  under  the 
latter  conditions;  it  might  therefore  be  supposed 
that  the  colouring  matter  is  more  soluble  in  the  fibre 
at  the  higher  temperature,  and  that  wool  which  had 
been  dyed  at  a  high  temperature  would  give  up  a 
portion  of  the  colour  which  it  had  absorbed  when 
it  cooled  down  again,  but  this  is  a  mode  of  behaviour 
not  met  with  in  practice. 

Now,  if  Witt's  theory  correctly  represents  the 
nature  of  the  dyeing  process,  we  should  expect  the 
laws  governing  the  distribution  of  a  substance  between 
two  immiscible  solvents  to  be  obeyed.  The  work 
of  Nernst  and  others  has  shown  us  that  when  a 
substance  has  the  same  molecular  weight  in  two 
solvents  the  ratio  of  concentrations  of  the  two 
solutions  after  distribution  of  the  substance  between 
the  solvents  is,  at  any  given  temperature,  independent 
both  of  the  quantity  of  solute  and  of  solvents,  and 
depends  only  on  the  solubilities  in  the  individual 
solvents.  If  the  molecular  weight  is  not  the  same 
in  the  two  solvents,  this  simple  ratio  is  departed 
from,  the  ratio  of  the  concentrations  of  the  two 
solutions  varying  with  the  amounts  of  substances 
used.  Even  in  such  a  case  it  is,  however,  possible 
to  obtain  an  expression  for  the  ratio  of  distribution 
of  the  substance  between  the  two  solvents.  Suppose 


54  THE  CHEMISTRY  OF   DYEING 

the  solvents  be  A  and  B,  and  that  the  molecular 
weight  of  a  substance  when  dissolved  in  the  latter 
solvent  be  n  times  as  great  as  when  dissolved  in  the 

C 
former;  then  —A-  will  be  constant  in  value,  CA  and 


CB  being  the  concentrations  of  the  dissolved  sub- 
stance in  the  two  solvents.  These  are  the  laws 
which  should  be  obeyed  if  the  dyeing  process  is 
simply  one  of  the  formation  of  a  solution  in  the 
fibre/ 

In  order  to  put  the  question  to  a  practical  test, 
Walker  and  Appleyard58  conducted  experiments  on 
the  dyeing  of  silk  with  picric  acid.  It  was  found 
that,  for  given  quantities  of  silk,  water,  and  picric 
acid,  it  was  immaterial  how  the  acid  was  distributed 
at  the  beginning  of  the  experiment,  the  same  ultimate 
equilibrium  being  arrived  at  whether  the  picric  acid 
was  only  dissolved  in  the  water,  all  contained  in  the 
silk,  or  partly  present  in  both  media  at  the  commence- 
ment of  the  experiment.  This  result  was  in  harmony 
with  the  requirements  of  the  solid  solution  theory, 
but  would  accord  equally  well  with  any  other 
theory  involving  the  establishment  of  a  condition 
of  equilibrium.  It  was  also  found  that  the  ratio  of 
distribution  of  the  acid  between  the  water  and  the 
silk  varied  with  the  quantities  involved,  but  that 
constant  values  were  obtained  when  a  formula  was 
employed  of  the  type  already  given,  the  result 
obtained  being 

Concentration  in  silk 

=  35'5- 


\/ Concentration  in  water 


SOLUTION  THEORY  UNTENABLE  55 

A  moment's  consideration  will  show  that  this  result 
is,  as  far  as  the  dyeing  of  silk  by  picric  acid  is 
concerned,  fatal  to  the  solid  solution  theory  of 
dyeing  as  proposed  by  Witt.  If  we  accept  the 
theory  as  true,  then  we  are  forced  to  the  conclusion 
that  the  molecular  weight  of  picric  acid  in  aqueous 
solution  is  nearly  three  times  as  great  as  when  the 
acid  is  contained  in  silk.  Such  an  idea  cannot,  of 
course,  be  entertained  for  a  moment,  for  we  know 
that,  owing  to  ionisation,  the  molecular  weight  of 
picric  acid  in  aqueous  solution  is  even  less  than 
corresponds  with  the  simple  formula  C6H2(N02)3OH, 
and  it  is  out  of  the  question  for  the  substance  to 
have  a  smaller  molecular  weight  in  a  less  active 
solvent  such  as  silk.  We  must  therefore  conclude 
that  picric  acid  does  not  exist  in  the  silk  in  a  state 
of  simple,  homogeneous  solution. 

Previous  to  the  publication  of  the  paper  to  which 
reference  has  just  been  made,  several  other  investiga- 
tions of  a  similar  character  had  been  carried  out  with 
varying  results.  Yon  Georgievics 26  concluded  that 
when  silk  is  dyed  with  Indigo-carmine  the  process 
is  analogous  to  solution,  but  his  results  really  lead 
to  conclusions  of  the  same  character  as  those  deduced 
from  W^alker  and  Appleyard's  experiments ;  in  a  later 
paper,  published  in  conjunction  with  Lowy,28  dealing 
with  the  distribution  of  Methylene  Blue  between 
water  and  mercerised  cotton,  von  Georgievics  came 
to  the  conclusion  that  his  results  were  incompatible 
with  the  solid  solution  theory.  Schmidt52  could 
obtain  no  constant  distribution  ratio,  and  considered 
dyeing  to  be  an  absorption  phenomenon. 


56  THE  CHEMISTRY  OF  DYEING 

Adsorption  Theory. — In  actions  of  this  kind  where 
one  substance  is  absorbed  by  another,  but  where  it 
is  evident  that  the  absorbed  substance  cannot  be 
in  a  condition  of  homogeneous  solution,  it  is  con- 
sidered that  the  surface  of  the  absorbing  substance 
plays  most  part  in  the  process  of  absorption,  and 
the  term  "adsorption"  is  made  use  of  to  describe 
such  cases  of  absorption.  According  to  the  results 
of  Walker,  von  Georgievics,  etc.,  we  should  conclude 
that  in  a  dyed  fibre  the  colouring  matter  is  not 
uniformly  distributed  throughout  the  fibre,  but  is 
collected  at  the  surface ;  in  other  words,  the  process 
is  one  of  adsorption.*  The  fact  that  the  surface  of 
a  fibre  appears  to  play  an  important  part  in  the 
dyeing  process  may  be  taken  as  showing  that  a  certain 
amount  of  what  may  be  called  mechanical  action 
does  really  enter  into  the  operation.  As  has  been 
indicated,  however,  we  cannot  explain  the  whole 
phenomena  of  dyeing  on  a  mechanical  basis.  Many 
investigations  have  been  carried  out  during  the  last 
fifteen  years  on  this  branch  of  the  subject  of  dyeing, 
and  the  results  obtained  support,  for  the  most  part, 
the  view  that  dyeing  cannot  be  regarded  simply  as 
the  formation  of  a  homogeneous  solution  of  the  dye 

*  To  speak  of  dyeing  as  a  surface  phenomenon  may  appear 
to  be  contrary  to  the  definition  of  "dyeing"  given  in  the 
introduction.  In  this  connection  it  is  only  necessary  to  point 
out  that  &  fabric  is  made  up  of  innumerable  individual  fibres, 
and  that,  owing  to  the  fabric  being  more  or  less  porous,  the 
solution  of  the  dye  can  penetrate  into  the  material,  so  that 
colour  can  be  absorbed  by  the  surface  of  a  fibre  in  the  interior 
of  the  fabric;  the  material  will  not,  therefore,  be  coloured 
only  on  its  surface. 


SURFACE  ACTION  57 

in  the  fibre.  Amongst  those  to  whom  we  are  in- 
debted for  further  knowledge  on  the  subject  may 
be  mentioned  Biltz,5  Hiibner,32  Freundlich  and 
Losev,21  Pelet  and  Grand,42  Schaposchnikoff,51  and 
Brown  and  M'Crae.10 

An  interesting  feature  of  Hubner's  experiments 
was  that  attention  was  particularly  directed  to  the 
influence  exerted  on  the  degree  and  rate  of  dyeing 
by  the  degree  of  division  of  a  fibre.  In  the  case 
of  cotton  a  quantity  of  yarn  was  first  thoroughly 
scoured  and  a  portion  then  disintegrated  by  treatment 
in  a  beating  engine.  On  afterwards  introducing 
equal  weights  of  the  beaten  and  unbeaten  fibre  into 
a  solution  of  Night  Blue,  it  was  found  that  while 
the  unbeaten  fibre  went  on  steadily  absorbing  the 
colouring  matter  during  a  period  of  seventy-two 
hours,  the  disintegrated  cotton  took  up  the  dye  very 
rapidly  during  the  first  hour,  and  very  little  more  was 
absorbed  during  the  succeeding  seventy-one  hours. 
Not  only  was  the  rate  of  absorption  greatly  accelerated 
by  reducing  the  state  of  division  of  the  fibre,  but  the 
actual  quantity  of  dye  absorbed  was  also  affected, 
about  twice  as  much  colour  being  taken  up  by  the 
disintegrated  fibre  as  was  absorbed  by  the  unbeaten 
cotton.  These  results  clearly  indicate  the  part  played 
by  the  surface  of  the  fibre  in  the  absorption  of  a 
colouring  matter  from  aqueous  solution,  for  of  course 
the  extent  of  surface  is  much  greater  in  the  case 
of  the  disintegrated  fibre  than  with  the  unbeaten 
cotton.  On  carrying  out  similar  experiments  with 
wool  it  was  found  that  the  rate  of  absorption  was 
very  considerably  increased,  but  there  was  no  increase 


58  THE  CHEMISTRY  OF  DYEING 

in  the  total  amount  of  dye  absorbed.  The  influence 
of  surface  was  further  shown  by  experiments  with 
threads  of  artificial  silk  of  different  diameters  and 
with  fine-  and  coarse-grained  emery;  in  both  cases 
the  proportion  of  dye  absorbed  by  the  fine  sample 
was  greater  than  that  absorbed  by  the  coarser  one. 

The  experiments  of  many  of  the  later  workers 
on  the  subject  have  been  partially  made  with  the 
idea  of  comparing  the  absorption  of  colouring  matters 
by  textile  fibres  with  absorption  by  inorganic  sub- 
stances such  as  animal  charcoal,  sand,  China  clay, 
aluminium  hydroxide,  etc.  The  results  clearly  show 
that  there  is  no  essential  difference  between  the 
absorption  of  dyes  by  fibres  and  by  inorganic 
substances,  and  that  in  both  cases  the  process  is 
subject  to  the  same  laws  and  appears  to  be  an 
instance  of  adsorption.  Freundlich  and  Losev  (loc. 
cit.)  found  that  the  extent  of  adsorption  is  inde- 
pendent of  the  nature  of  the  adsorbent,  a  dye  which 
was  strongly  adsorbed  by  charcoal  also  being  strongly 
adsorbed  by  the  textile  fibres.  Basic  dyes  were 
decomposed  into  acid  and  base  both  in  the  presence 
of  charcoal  and  of  fibres,  the  acid  remaining  in 
solution  and  the  colour  base  being  adsorbed,  probably 
in  a  polymeric  form.  In  the  case  of  mordant  dyeing 
the  adsorption  theory  can  still  be  applied,  the  only 
difference  being  that  it  is  the  mordant  which  under- 
goes adsorption. 

Although  the  bulk  of  the  evidence  of  a  quantitative 
character  has  been  shown  to  be  unfavourable  to  the 
simple  solid  solution  theory,  there  have  been  several 
cases  put  on  record  of  experimental  results  which 


ANOTHER  SOLUTION  THEORY      59 

appear  to  be  in  harmony  with  that  theory.  Brown 
and  M'Crae  (loc.  cit.)  showed  that  when  wool  is 
dyed  with  Acid  Magenta  in  the  presence  of  sulphuric 
acid  and  with  Chrysoidine  FF,  the  ratio  of  distribu- 
tion of  the  colouring  matter  between  the  fibre  and 
the  water  is  practically  independent  of  the  concentra- 
tion of  the  solution,  so  that  in  the  case  of  these 
dyes  it  would  appear  as  if  the  dyeing  process  is  of 
a  nature  similar  to  solution.  Sisley54  also  obtained 
results  which  he  considered  to  be  favourable  to 
Witt's  theory. 

In  concluding  this  section  reference  must  be  made 
to  another  idea  regarding  solution  to  which  expression 
was  given  by  Weber.59  It  has  already  been  explained 
that  this  chemist  regarded  the  dyeing  of  fibres  with 
the  acid  and  basic  dyes  as  a  process  of  a  chemical 
nature.  He  recognised  that  the  absorption  of  the 
direct  cotton  dyes  by  fibres  of  different  chemical 
natures  could  not  be  explained  in  the  same  manner, 
and  he  was  led  to  the  conclusion  that  in  the  case 
of  colouring  matters  of  this  class  the  process  was  one 
of  solution.  Whereas,  however,  Witt  considered  the 
dyestuff  to  be  dissolved  by  the  substance  of  the  fibre 
itself,  Weber  was  of  the  opinion  that  the  colouring 
matter  dissolved  in  the  water  contained  in  the  pores 
or  intercellular  spaces  of  the  fibre.  In  support  of 
this  view  he  pointed  out  that  cellulose  dinitrate 
can  be  manufactured  in  such  a  manner  that  it  shows 
little  structural  difference,  when  examined  by  means 
of  the  microscope,  from  ordinary  cotton,  and  these 
nitrated  fibres  can  be  dyed  with  the  benzidine  colours 
in  just  the  same  way  as  ordinary  cotton.  If  the 


60  THE  CHEMISTRY  OF  DYEING 

nitrated  cotton  is  dissolved  in  acetone,  and  the  solvent 
allowed  to  evaporate,  a  film  is  left  which  is  devoid 
of  structure  and  which  contains  no  water.  In  this 
form  the  nitrated  cotton  cannot  be  dyed  with  the 
benzidine  colouring  matters.  Additional  support  for 
this  view  was  obtained  by  Weber  from  a  microscopical 
examination  of  dyed  fibres,  when  in  many  cases  the 
cell  walls  appeared  colourless,  while  the  lumen  was 
filled  with  colouring  matter. 

Other  Theories  of  Dyeing. 

An  electrical  explanation  of  the  attraction  of 
fibres  for  colouring  matters  was  given  by  Gee  and 
Harrison.23  In  the  case  of  basic  and  acid  dyes,  the 
colour  base  of  the  former  and  the  acid  of  the  latter 
carry  a  positive  and  negative  charge  respectively 
derived  from  the  ionisation  of  the  dyestuff.  When 
wool  or  silk  is  immersed  in  water  the  fibre  becomes 
negatively  charged,  and  it  is  quite  natural  therefore 
that  when  such  a  fibre  is  immersed  in  a  neutral 
solution  of  a  basic  dye  there  will  be  an  attraction 
between  the  negatively  charged  fibre  and  the  positively 
charged  colour  base.  When  the  fibres  are  placed 
in  an  acid  solution  instead  of  in  pure  water  they 
become  positively  charged;  such  a  change  in  the 
kind  of  electrification  with  the  nature  of  the  solution 
in  which  the  substance  is  immersed  is  frequently 
noticed  with  substances  of  a  colloidal  character. 
Under  these  changed  conditions  the  fibre  will  now 
have  an  affinity  for  the  negatively  charged  acid  of 
acid  colouring  matters,  while  the  attraction  for  basic 


AN  ELECTRICAL  THEORY  61 

dyes  will  now  be  less  than  before  in  view  of  the 
fact  that  the  two  are  now  similarly  charged.  This 
latter  deduction  is  quite  in  keeping  with  the  results 
obtained  in  practice,  for  it  is  well  known  that  the 
basic  dyes  are  much  less  readily  absorbed  from  acid 
solution  than  from  a  neutral  one,  and  this  device 
is  sometimes  resorted  to  in  order  to  regulate  the 
rate  at  which  a  basic  colouring  matter  is  absorbed. 
This  theory  also  accounts  in  quite  a  satisfactory 
manner  for  the  much  smaller  attraction  shown  for 
dyestuffs  of  the  acid  and  basic  groups  by  a  fibre 
such  as  cotton.  The  potential  difference  between 
wool  and  water  is  equal  on  the  average  to  0-91  volt, 
while  with  cotton  the  average  value  of  the  potential 
difference  is  only  0-06  volt.  Owing  to  this  much 
smaller  charge  upon  the  cotton,  it  will  be  readily 
understood  why  the  fibre  should  have  much  less 
attraction  for  colouring  matters  than  is  shown  by 
wool.  While  this  theory  gives  an  apparently  satis- 
factory explanation  of  the  causes  underlying  the 
affinity  of  fibres  for  dyestuffs,  it  does  not  explain 
how  the  colouring  matter  becomes  fixed  upon  the 
fibre;  for  of  course  after  the  dye  and  fibre  come 
into  actual  contact,  it  must  naturally  be  assumed 
that  their  electrical  charges  will  neutralise  each  other. 
A  somewhat  similar  view  was  taken  by  Feilmann,20 
according  to  whom  the  coloured  ion  formed  from 
the  dyestuff  was  attracted  by  the  oppositely  charged 
fibre,  and  penetrated  the  latter  more  or  less  deeply. 
The  absorbed  ion  was  supposed  to  be  retained,  either 
because  the  fibre  acted  as  a  protective  colloid,  or 
because  of  chemical  action  taking  place  between  the 


62  THE  CHEMISTRY  OF  DYEING 

ion  and  the  fibre.  It  will  be  seen  that  this  idea 
goes  a  little  further  than  that  of  Gee  and  Harrison, 
inasmuch  as  it  gives  a  possible  explanation  of  the 
means  of  fixation  of  the  dye  on  the  fibre. 

Colloidal  Theory. — Of  late  years  a  large  and  ever- 
increasing  amount  of  attention  has  been  given  to  the 
properties  and  behaviour  of  colloids.  The  fibres 
themselves  are  of  course  substances  of  a  colloidal 
character,  and  it  has  been  shown  in  an  earlier  portion 
of  the  book  that  many  dyes  when  in  solution  probably 
exist  in  the  colloidal  form.  In  view  of  these  facts 
it  is  not  surprising  that  attempts  have  been  made 
to  explain  the  dyeing  process  as  a  colloidal 
phenomenon.  One  of  the  first  to  take  up  this  view 
of  the  matter  was  Krafft.39  He  pointed  out  that  the 
substances  commonly  employed  as  mordants,  such  as 
the  hydroxides  of  iron,  aluminium  and '  chromium, 
tannic  acid,  soap,  etc.,  are  all  of  a  colloidal  nature, 
and  suggested  that  the  use  of  such  substances  was 
in  many  cases  necessary  in  order  to  combine  with 
dyes  of  low  molecular  weight,  which  under  normal 
conditions  only  existed  in  solution  to  a  small  degree 
in  the  colloidal  condition,  so  as  to  form  a  more  highly 
colloidal  compound  capable  of  being  fixed  on  the 
fibre.  As  for  the  direct  cotton  dyestuffs,  these  were 
supposed  to  exist  in  the  colloidal  condition  to  a  much 
greater  extent  than  the  dyes  of  the  acid  and  basic 
groups,  and  no  assistance  was  therefore  necessary  in 
order  to  fix  the  colouring  matter.  Of  course  this 
explanation  of  the  dyeing  process,  according  to  which 
dyeing  is  simply  a  precipitation  of  colloidal  substances 
on  or  in  the  fibre,  only  applies  to  the  dyeing  of  cotton, 


THE  COLLOIDAL  THEORY  63 

for  wool  and  silk  do  not  require  any  mordant  in 
order  to  fix  the  acid  or  basic  dyestufFs.  Pelet- 
Jolivet  and  Andersen4344  were  also  in  favour  of  a 
colloidal  theory  of  dyeing. 

The  view  was  expressed  by  several  of  the  supporters 
of  the  adsorption  theory  that  the  substance  actually 
fixed  on  the  fibre  was  in  the  colloidal  condition. 
Freundlich  and  Losev,21  for  example,  considered  that 
in  the  case  of  wool  dyed  with  a  basic  colouring 
matter,  the  colour  base  was  adsorbed  by  the  fibre 
in  a  colloidal  form  insoluble  in  water ;  and  they  also 
considered  that  in  some  instances  dyeing  might  be 
due  to  the  formation  of  a  colloidal  compound  between 
the  fibre  and  the  dyestuff.  Pelet  and  Grand42  also 
considered  that  dyeing  is  due  to  the  precipitation  of 
colloids  on  the  fibre,  and  that  the  salts  added  to  the 
dyebath  assist  in  this  precipitation.  Linder  and 
Picton 41  from  experiments  made  with  Methyl  Violet, 
Magenta,  Soluble  Blue,  etc.,  were  led  to  connect  the 
phenomena  of  dyeing  with  the  electrical  charges 
which,  as  is  well  known,  many  substances  carry 
when  existing  in  a  state  of  colloidal  solution.  They 
found  that  whereas  a  colloidal  solution  of  ferric 
hydroxide  is  precipitated  by  the  addition  of  a 
solution  of  Soluble  Blue,  no  such  action  takes  place 
on  the  addition  of  Methyl  Violet  to  the  ferric 
hydroxide  solution;  on  the  other  hand,  a  colloidal 
solution  of  arsenious  sulphide  is  precipitated  by 
solutions  of  basic  dyes  like  Methyl  Violet  and 
Magenta  but  not  by  Soluble  Blue,  an  acid  colouring 
matter.  The  reason  is  that  the  charges  on  the  ferric 
hydroxide  and  the  basic  dyes  are  of  a  similar  nature, 


64  THE  CHEMISTRY  OF  DYEING 

these  substances  carrying  positive  charges,  while  the 
arsenious  sulphide  and  the  Soluble  Blue  carry  negative 
charges;  as  is  well  known,  when  two  oppositely 
charged  colloidal  solutions  are  mixed  together  they 
precipitate  each  other.  A  difference  was  noticed, 
however,  between  the  precipitation  of  the  ferric 
hydroxide  by  the  acid  dye  and  by  means  of  an 
electrolyte  such  as  ammonium  sulphate.  In  the 
latter  case  a  definite  amount  of  the  salt  is  required 
to  completely  precipitate  the  colloid,  and  any  excess 
which  may  be  added  remains  in  solution.  With  /the 
dye,  however,  a  different  result  was  observed,  for 
even  after  the  ferric  hydroxide  had  been  completely 
thrown  out  of  colloidal  solution  it  continued  to  take 
up  the  colouring  matter  as  a  whole.  Linder  and 
Picton  explained  this  on  the  supposition  that  a 
certain  portion  of  the  original  charge  was  retained 
by  the  ferric  hydroxide  even  after  it  had  been 
coagulated  by  the  addition  of  the  dyestuff,  and  that 
by  virtue  of  this  residual  charge  it  continued  to  take 
up  additional  quantities  of  the  dye.  The  same  kind 
of  action  was  considered  to  take  place  during  the 
ordinary  process  of  dyeing,  the  place  of  the  inorganic 
colloid  being  taken  by  the  fibre.  They  were  accord- 
ingly of  the  opinion  that  the  first  stage  of  the  dyeing 
process  consisted  in  the  separation  of  insoluble 
derivatives  of  the  dye  having  a  feeble  charge,  these 
substances  being  produced  as  the  result  of  interaction 
between  the  dyestuff  and  the  fibre ;  the  second  part  of 
the  process  was  an  attraction  between  this  coagulum 
and  the  remaining  dyestuff,  the  particles  of  which 
were  retained  as  a  whole. 


EFFECT  OF  PROTECTIVE  COLLOIDS         65 

It  will  be  noticed  there  is  a  certain  degree  of 
similarity  between  these  views  and  those  expressed 
later  by  Gee  and  Harrison  (loc.  cit.),  inasmuch  as  both 
pairs  of  observers  consider  the  electrical  condition 
of  the  fibre  and  the  dyestuff  to  play  an  important 
part  in  the  process ;  the  differences  between  the  two 
were  that  Linder  and  Picton  regarded  the  matter 
exclusively  from  the  point  of  view  that  the  substances 
with  which  they  were  concerned  all  existed  in  the 
colloidal  condition,  and  that  they  were  led  to  regard 
the  taking  up  of  a  dye  as  proceeding  in  two  stages. 

Alexander l  showed  that  various  protective  colloids, 
such  as  gelatin  and  gum  arabic,  when  added  to  a 
solution  of  Benzopurpurin  had  the  effect  of  causing 
the  mixture  to  behave  in  exactly  the  same  manner 
when  treated  with  acid  as  wool  or  silk  which  had 
been  dyed  with  the  same  colouring  matter.  The 
addition  of  a  dilute  solution  of  hydrochloric  acid  to 
a  solution  of  Benzopurpurin  causes  the  colour  to 
change  from  red  to  blue,  while  on  the  addition  of 
a  more  concentrated  solution  of  acid  the  coagulated 
dark  blue  colour  acid  separates  out.  If  a  similar 
solution  of  Benzopurpurin  is  first  mixed  with  one 
of  the  above-mentioned  colloids  the  colour  is  only 
changed  to  claret  red  or  chocolate  brown  on  the 
addition  of  hydrochloric  acid,  the  colour  depending 
on  the  concentration  of  the  added  acid,  and  even 
when  concentrated  acid  is  used  no  precipitate  is  pro- 
duced. These  changes  were  investigated  by  means 
of  the  ultramicroscope ;  it  was  found  that  no  change 
resulted  in  the  presence  of  the  colloid,  unless  the 
added  acid  was  of  sufficient  concentration  to  cause 

E 


66  THE  CHEMISTRY  OF  DYEING 

agglutination  of  ultramicrons  into  small  groups.  As 
regards  the  protective  action  of  different  colloids, 
it  was  found  that  gelatin  was  much  more  effective 
than  gum  arabic,  and  that  starch  had  very  little 
protective  action.  In  the  light  of  these  results,  it 
was  considered  that  the  difference  in  colour  changes, 
which  result  when  different  fibres  dyed  with  Benzo- 
purpurin  are  immersed  in  dilute  acid,  is  due  to  the 
difference  in  protective  action  of  the  fibre  on  the 
adsorbed  colouring  matter. 

Alexander  pointed  out  that  in  considering  the 
nature  of  the  combination  between  the  fibre  and 
the  dye,  it  is  necessary  to  bear  in  mind  the  state 
of  subdivision  of  both  substances.  The  usual  effect 
of  increase  of  temperature  and  of  the  presence  of 
dilute  alkalis  is  to  cause  subdivision  of  the  particles 
of  a  substance,  and  it  is  not  surprising  therefore  that 
agencies  such  as  these  should  bring  about  a  closer 
union  between  the  fibre  and  the  dye,  such  as  Dreaper 
and  Wilson1718  had  shown  to  exist.  Dreaper  and 
Wilson,18  in  fact,  consider  that  the  temperature  of 
the  dyebath  is  the  main  factor  in  the  actual  fixation 
of  the  dye  on  the  fibre,  and  that  though  in  the 
application  of  acid  dyes  the  presence  of  acid  in  the 
bath  may  lead  to  a  larger  amount  of  dye  being  taken 
up  than  would  be  the  case  from  a  neutral  solution, 
yet  the  acid  plays  no  part  in  the  fixing  of  the  colour. 
Dreaper  and  Wilson  have  shown  that  it  is  possible 
to  apply  acid  dyes  from  an  alkaline  solution ;  under 
such  conditions,  of  course,  the  substance  actually 
absorbed  by  the  fibre  can  scarcely  be  the  free  colour 
acid.  They  consider  that  the  sodium  carbonate  used 


EFFECT  OF  ALKALI  ON  A  DYE  67 

in  the  bath  may  act  as  a  salt  rather  than  an  alkali, 
and  that  it  may  affect  the  state  of  aggregation  of 
the  dissolved  colouring  matter  and  bring  into  play 
conditions  more  favourable  to  colloidal  action. 

Alexander  (loc.  cit.)  studied  by  the  aid  of  the  ultra- 
microscope  the  effect  produced  when  alkali  is  added 
to  a  solution  of  Acid  Anthracene  Red.  The  appear- 
ance of  the  aqueous  solution  of  this  colouring  matter 
would  lead  one  to  conclude  that  a  portion  of  the 
dye  at  any  rate  is  in  a  very  fine  state  of  subdivision, 
if  not  in  true  solution.  When  the  dye  was  dissolved 
in  a  solution  of  sodium  carbonate  of  N/20  concentra- 
tion a  certain  amount  of  coagulation  took  place ;  but 
this  only  represented  part  of  the  total  quantity  of 
dye  actually  present,  for  the  alkaline  solution,  on 
standing,  was  found  to  give  only  a  small  amount  of 
sediment,  and  it  also  gave  a  marked  Tyndall  effect 
when  examined  with  the  ultramicroscope.  When 
a  more  concentrated  solution  of  sodium  carbonate 
was  employed,  the  bulk  of  the  dye  was  precipitated ; 
but  this  precipitate  dissolved  on  being  heated,  and 
appeared  again  when  the  liquid  was  allowed  to  cool. 
One  would  infer  from  this  experiment  that  the  state 
of  a  dye  in  solution  is  modified  both  by  alkali  and 
by  change  of  temperature,  and  Alexander  considered 
that  these  factors  might  explain  the  fixation  of  the 
dye  which  would,  in  accordance  with  the  ordinary 
conditions  of  dyeing,  be  absorbed  at  a  high  temperature 
and  might  be  precipitated  again  on  cooling,  or  be 
flocculated  by  adsorbed  alkali. 


68  THE  CHEMISTRY  OF  DYEING 

The  Divided  Nature  of  the  Dyeing  Process. 

It  may  have  been  noticed  that  in  most  of  the  ideas 
to  which  expression  has  been  given  in  the  preceding 
pages,  the  dyeing  process  has  been  regarded  as  simple 
in  character,  and  the  supporters  of  the  different 
theories  have,  for  the  most  part,  attempted  to  explain 
all  the  phenomena  of  dyeing  as  resulting  from  a 
single  operation.  This  it  is  which  has  led  to  the 
various  difficulties  which  have  confronted  the 
supporters  of  the  different  theories,  and  which  have 
led  some  chemists  such  as  Weber 59  and  Brown  and 
M'Crae10  to  the  conclusion  that  no  one  theory  can 
possibly  explain  all  cases  of  dyeing,  and  that  it  may 
be  necessary  to  adopt  one  theory  to  explain  the 
dyeing  of  one  fibre,  and  another  theory  to  explain 
the  dyeing  of  a  second  fibre.  This  would  certainly 
appear  to  be  the  inevitable  conclusion  if  we  are  to 
regard  the  dyeing  process  as  simple  in  character, 
that  is,  as  involving  one  operation  only.  All  the 
theories  have  a  certain  amount  of  experimental 
evidence  which  can  be  cited  in  support  of  them ; 
but  they  are  more  or  less  antagonistic  in  character, 
and  no  one  of  them  will  explain  the  whole  of 
the  phenomena  associated  with  the  subject  of 
dyeing. 

It  is  the  aim  of  the  author  in  the  present  section 
to  show  that  all  these  opposing  theories  may  be 
true  as  partial  explanations  of  certain  cases  of  dyeing, 
and  that  they  may  be  linked  together  in  one  general 
theory  of  the  dyeing  process.  In  order  to  arrive 
that  such  a  result  it  is  necessary  to  assume  at  the 


FIRST  STAGE  OF  DYEING  69 

operation  of  dyeing  takes  place  in  two  distinct  stages. 
Dreaper,16  Cross  and  Bevan,u  Lewis,40  Zacharias,61  and 
Fahrion19  have  all  expressed  themselves  in  favour 
of  regarding  the  dyeing  process  as  having  a  dual 
character,  and  in  the  opinion  of  the  author  it  is 
the  most  rational  view  to  take  of  the  matter. 

In  the  first  part  of  the  process,  which  may  be 
called  the  absorption  stage,  the  dyestuff  is  simply 
absorbed  from  solution.  This  absorption  was  regarded 
by  Zacharias  as  being  brought  about  by  the  diffusion 
of  the  dissolved  dye  from  the  aqueous  solution  into 
the  fibre,  and  no  particular  attraction  between  the 
fibre  and  the  dye  needs  to  be  invoked  to  explain  this 
diffusion,  although  the  process  may  be  assisted  by 
electrical  attraction.  It  is  the  process  which  takes 
place  naturally  when  a  layer  of  water  is  placed  over 
a  solution  of  a  dissolved  salt,  or  when  a  liquid, 
immiscible  with  water,  is  placed  in  contact  with  an 
aqueous  solution  of  a  substance  which  is  also  soluble 
in  the  second  solvent.  Just  as  in  those  cases  diffusion 
is  a  slow  process  so  that  homogeneous  solutions  are 
only  obtained  after  the  lapse  of  a  considerable  time, 
so  also  the  process  of  diffusion  into  the  fibre  will 
take  place  slowly,  and  although  the  dye  may  be  said 
to  be  dissolved  by  the  fibre,  yet,  generally  speaking, 
the  solute  will  not  be  uniformly  distributed  through- 
out the  fibre  but,  because  of  this  slow  diffusion,  will 
be  present  in  largest  quantity  at  the  surface ;  in  other 
words,  the  process  will  appear  as  an  adsorption 
phenomenon. 

The  fibres  are  hygroscopic  colloids,  and  absorb 
other  substances  in  accordance  with  certain  general 


70  THE  CHEMISTRY  OF  DYEING 

laws.     All  colloidal  substances  absorb  others  accord- 
ing to  the  law  embodied  in  the  expression 


where  C1  and  C2  represent  the  concentration  in  the 
aqueous  and  in  the  other  phase  respectively  at  the 
end  of  the  absorption,  k  is  a  constant  and  v  a  constant 
coefficient,  which  may  have  a  value  either  greater 
or  less  than  or  equal  to  unity. 

It  was  pointed  out  by  Zacharias  that  this  formula 
may  be  deduced  mathematically  from  the  laws  of 
diffusion,  and  accordingly  the  dyeing  of  textile  fibres 
and  also  of  other  substances  such  as  charcoal,  alu- 
minium hydroxide,  etc.,  and  the  quantitative  results 
obtained  in  these  processes  can  all  be  explained  on 
this  basis  of  diffusion  of  the  dissolved  dye.  The 
formula  is  identical  with  that  representing  the 
distribution  of  a  substance  between  immiscible 
solvents  where  homogeneous  solutions  are  formed, 
but  it  has  already  been  shown  what  difficulties  are 
met  with  if  we  regard  the  matter  from  this  latter 
standpoint. 

So  far  we  have  only  considered  the  first  stage  of 
dyeing;  the  second  part  of  the  process  is  the  fixation 
of  the  colouring  matter.  There  could  be  no  permanent 
dyeing  without  this  second  operation.  Here  we 
have  to  recognise  the  fact  that  dyes  may  be  fixed 
on  fibres  as  the  result  of  the  operation  of  different 
forces. 

In  some  cases  it  seems  probable  that  a  certain 
amount  of  chemical  change  takes  place,  either  between 
the  fibre  and  the  dyestuff  or  in  the  structure  of  the 


SECOND  STAGE  OF  DYEING  71 

latter  compound.  Bayliss 2  has  put  on  record  a  case 
of  dyeing,  where  a  process  of  absorption  was  un- 
doubtedly succeeded  by  one  of  chemical  action. 
When  a  dilute  colloidal  solution  of  the  blue  colour 
acid  of  Congo  Red  is  mixed  with  well-washed 
aluminium  hydroxide,  the  latter  absorbs  the  colour 
acid  and  acquires  a  blue  colour;  this  is  adsorption. 
On  suspending  the  blue  precipitate  in  water  and 
warming  the  mixture,  the  colour  changes  to  red 
owing  to  the  formation  of  an  aluminium  salt, 
chemical  action  taking  place  between  the  colour 
acid  and  the  basic  hydroxide. 

No  doubt  similar  chemical  changes  take  place  in 
the  dyeing  of  textile  fibres.  Cross  and  Bevan 13  have, 
in  fact,  described  a  very  good  example  of  a  fibre 
being  dyed  as  the  result  of  a  double  process.  If  jute 
is  immersed  in  a  solution  of  ferric  ferrocyanide  it 
very  soon  becomes  dyed  blue  owing  to  the  production 
of  Prussian  Blue.  In  this  case  the  substance  absorbed 
is  the  soluble  ferric  salt;  this  then  reacts  with  the 
jute  substance,  the  unsaturated  lignone  groups  of 
which  are  oxidised,  and  as  a  result  of  this  action 
insoluble  ferrous  ferricyanide  is  produced  and  fixed 
in  the  fibre. 

In  other  cases  the  dye  may  have  been  originally 
present  in  the  dyebath  in  a  condition  of  colloidal 
solution,  and  the  fixation  of  the  colouring  matter 
may  be  due  to  the  precipitation  of  the  colloid. 
Possibly,  also,  a  certain  amount  of  colour  may  be 
fixed  as  the  result  of  adhesion  between  the  fibre 
and  dyestuff. 

This   theory   of    the   dual   nature   of    the   dyeing 


72  THE  CHEMISTRY  OF  DYEING 

process  applies  to  cases  of  mordant  dyeing,  to  the 
developed  dyes,  and  those,  such  as  Indigo,  which 
are  applied  by  the  vat  method,  equally  well  as  to 
all  cases  of  substantive  dyeing.  In  all  cases  we  have 
a  first  process  of  absorption;  in  the  case  of  the 
mordant  dyes  it  is  usually  the  mordant  which  is 
taken  up  in  this  manner,  and  the  mordant  is  then 
fixed  in  an  insoluble  form.  The  colouring  matter  is 
then  absorbed  from  its  solution  and  reacts  with  the 
previously  fixed  mordant  to  form  the  colour  lake. 
The  dyeing  of  cotton  with  lead  chromate  and  with 
the  azo  dyes  like  paranitraniline  red  formed  directly 
on  the  fibre  also  takes  place  in  two  stages,  while  in 
the  case  of  the  vat  dyes  we  have  first  the  absorption 
of  the  soluble  leuco  compound,  and  second  the  fixation 
of  the  dye  in  the  insoluble  form  owing  to  the 
oxidation  of  the  leuco  compound. 

As  for  the  fact  that  with  some  colouring  matters 
the  dyebath  is  practically  completely  exhausted,  while 
with  others  this  is  never  the  case,  the  probable 
explanation  is  that  the  degree  to  which  the  bath  is 
exhausted  depends  both  upon  the  rate  of  absorption 
and  on  the  rate  of  fixation  of  the  dye.  With  a  dye 
which  is  fixed  readily  and  which  diffuses  into  the 
fibre  with  considerable  velocity,  there  appears  no 
reason  why  the  dyebath  should  not  be  practically 
exhausted  in  the  period  usually  occupied  by  the 
dyeing  process. 

This  method  of  regarding  the  dyeing  process  also 
provides  an  answer  to  the  objections  levelled  against 
the  solid  solution  theory  with  respect  to  the  non- 
reversible  character  of  the  dyeing  process.  Were 


CAUSE  OF  PAST  CONFUSION  73 

dyeing  nothing  but  an  absorption,  an  extraction  of 
an  aqueous  solution  by  means  of  the  immiscible 
solvent,  the  fibre,  there  would  be  no  reason  why  the 
process  should  nob  be  reversible.  When,  however, 
the  dye  becomes  fixed,  we  change  its  properties  either 
by  forming  an  insoluble  compound  or  by  convert- 
ing it  into  a  modification  insoluble  in  water,  and  so 
reversibility  of  the  process  ceases  to  be  possible. 

In  concluding  this  study  of  the  dyeing  process,  the 
author  would  point  out  that  some  at  least  of  the 
diversity  of  opinion  expressed  in  the  past  has  been 
due  to  the  fact  that  a  certain  amount  of  confusion 
prevailed  as  to  what  really  constituted  the  dyeing 
process.  Some  workers  appeared  only  to  regard  the 
method  of  fixation  of  the  dye  on  the  fibre;  others 
paid  more  attention  to  the  process  of  absorption. 
As  has  been  shown,  the  dyeing  process  embraces  both 
absorption  and  fixation,  and  every  example  of  dyeing 
can  be  shown  to  include  these  two  operations.  This 
may  therefore  be  called  a  general  theory  or  explana- 
tion of  the  nature  of  the  dyeing  process.  It  is  true 
that  different  methods  of  fixation  of  the  dyes  are 
recognised,  and  some  may  regard  this  as  an  objection 
to  a  general  theory,  and  as  not  much  of  an  advance, 
if  any,  over  the  position  that  no  one  theory  of  dyeing 
can  be  applied  to  all  cases,  but  that  in  some  instances 
the  phenomena  are  to  be  explained  on  one  basis,  and 
in  other  cases  on  a  different  one.  This  difference 
in  the  mode  of  fixation  is,  however,  a  matter  of  minor 
importance  due  to  the  great  differences  in  properties 
of  the  fibres  and  dyestuffs ;  to  have  reached  the  stage 
of  being  able  to  say  that  no  matter  what  the  fibre, 


74  THE  CHEMISTRY  OF  DYEING 

no  matter  what  the  dyestuff  may  be,  the  process  of 
dyeing  the  one  with  the  other  can  always  be  divided 
into  the  two  stages  of  absorption  and  fixation  is 
certainly  to  have  made  a  very  considerable  advance 
in  our  knowledge  of  the  dyeing  process. 


BIBLIOGRAPHY 

1  Alexander,  Journ.  Soc.  Chem.  Ind.,  1911,  30,  517. 

2  Bayliss,  Zeit.  Chem.  Ind.  Roll.,  1908,  3,  224. 

3  Bayliss,  Proc.  Roy.  Soc.,  1909,  81,  B,  269. 

4  Bentz  and  Farrell,  Journ.  Soc.  Chem.  Ind.,  1897,  16,  406. 

5  Biltz,  Chem.  Zentr.,  1905,  2,  524. 

0  Biltz  and  Pfenning,  Gedankboek  aan  van  Bemmelen,  1910, 
108. 

7  Biltz  and  Pfenning,  Zeit.  physikal.  Chem.,  1911,  77,  91. 

8  Biltz  and  Vegesack,  Zeit.  pbysikal.  Chem.,  1910,  73.  481. 

9  Binz  and  Schroeter,  Ber.,  1902,  35,  4225  ;  1903,  36,  3008. 

10  Brown  and  M'Crae,  Journ.  Soc.  Chem.  Ind.,  1901, 20, 1092. 

11  Champion,  Compt.  rend.,  72,  330. 

12  Cross  and  Bevan,  Researches  on  Cellulose. 

13  Cross  and  Bevan,  Journ.  Soc.  Chem.  Ind.,  1893,  12,  104. 

14  Cross  and  Bevan,  Journ.  Soc.  Chem.  Ind.,  1894,  13,  354. 

15  Donnan  and  Harris,  Chem.  Soc.  Trans.,  1911,  99,  1554. 

10  Dreaper,  Journ.  Soc.  Chem.  Ind.,  1894,  13,  95 ;  1905,  24, 
223. 

17  Dreaper  and  Wilson,  Journ.  Soc.  Chem.  Ind.,  1907,26,  667. 

18  Dreaper  and  Wilson,  Journ.  Soc.  Chem.  Ind.,  1910,  29, 

1432. 

19  Fahrion,  Chem.  Zeit.,  1908,  32,  357. 

20  Feilmann,  Journ.  Soc.  Dyers  and  Col.,  1909,  25,  158. 

21  Freundlich  and  Losev,  Zeit.  physikal.  Chem.,   1907,  59 

284. 

22  Freundlich  and  Neumann,  Zeit.  Chem.  Ind.  Roll.,  1908, 

3,80. 

23  Gee  and  Harrison,  Trans.  Faraday  Soc.,  1910, 


76  BIBLIOGRAPHY 

2i  Gelmo  and  Suida,  Monatsh.  Chem.,  1905,  26,  855 ;  1906, 
27,  1193. 

25  Georgievics,  G.  von,  Mitt.  Tech.  Gew.  Museum  in  Wien, 

4,  205  and  349. 

26  Georgievics,  von,  Monatsh.  Chem.,  1894,  15,  705. 

27  Georgievics,  von,  Zeit.  angew.  Chem.,  1902,  16,  [24],  574. 

28  Georgievics,  von,  and  Lowy,  Monatsh.  Chem.,  1895,   16, 

345. 

29  Gillet,  Kev.  Gen.  Mat.  Col.,  1900,  4,  [6],  183. 

30  Gillet,  Rev.  Gen.  Mat.  Col.,  1900,  4,  [46],  305. 

31  Hantzsch  and  Osswald,  Ber.,  1900,  33,  303. 

32  Hiibner,  Chem.  Soc.  Trans.,  1907,  91,  1057. 

33  Hwass,  Lehne's  Farb-Zeit.,  1890-91,  224. 

34  Knecht,  Journ.  Soc.  Dyers  and  Col.,  1888,  104. 

35  Knecht,  Ber.,  1888,  21,  1556. 

36  Knecht,  Ber.,  1904,  37,  3479. 

37  Knecht  and  Apple-yard,   Journ.   Soc.    Dyers    and    Col., 

1889,  71. 

38  Knecht  and  Batey,  Journ.   Soc.  Dyers  and  Col.,  1909, 

25,  194. 

39  Krafft,  Ber.,  1899,  32,  1608. 

40  Lewis,  Phil.  Mag.,  1908,  [vi],  15,  499. 

41  Linder  and  Picton,  Chem.  Soc.  Trans.,  1905,  87,  1906. 

42  Pelet  and  Grand,  Rev.  Gen.  Mat.  Col.,  1907,  11,  225. 

43  Pelet-Jolivet,  Compt.  rend.,  1907,  145,  1182. 

44  Pelet-Jolivet  and  Andersen,  Compt.  rend.,  1907,  145,  1340. 

45  Pelet-Jolivet  and  Wild,  Zeit.  Chem.  Ind.  Koll.,  1908,  3, 

174. 

46  Perger,  von,  Lehne's  Farb-Zeit.,  1890-91,  356  and  371. 

47  Pfeffer,  Chem.  Zeit.,  10,  1259. 

48  Prud'homme,  Rev.  Gen.  Mat.  Col.,  1900,  4,  [6],  189. 

49  Prud'homme,  Rev.  Gen.  Mat.  Col.,  1900,  4,  [41],  156. 

50  Rosenstiehl,  Compt.  rend.,  1909, 149,  396. 

61  Schaposchnikoff,  Zeit.  physikal.  Chem.,  1911,  78,  209. 
52  Schmidt,  Zeit.  physikal.  Chem.,  1894,  15,  56. 
63  Sisley,  Rev.  G&i.  Mat.  Col.,  1900,  4,  [6],  180. 
54  Sisley,  Bull.  Soc.  Chim.,  1902,  27,  901. 
r*  Spohn,  Dingl.  polyt.  J.,  1893,  287,  210. 


BIBLIOGRAPHY  77 

56  Suida,  Z.  Farb.-Ind.,  1907,  6,  41 ;  Z.  physiol. 'Chexn.,  1910, 

68,  381. 

57  Vignon,  Compt.  rend.,  112,  487  and  623 ;  148,  1195 ;  Bull. 

Soc.  Ind.  Mulhouse,  1892,  563 ;  1893,  407. 
:>8  Walker  and  Appleyard,  Chem.  Soc.  Trans.,  1896,  69,  1334. 
;>!)  Weber,  Journ.  Soc.  Chem.  Ind.,  1894,  13,  120. 

00  Witt,  Farberzeitung,  1890-91,  1. 

01  Zacharias,  Farberzeitung,  1901,  12,  149  and  165. 


Readers  who  desire  to  study  the  subject  in  greater  detail, 
but  who  do  not  wish  or  have  not  the  facilities  to  consult  the 
original  papers,  may  be  referred  to  the  following  books,  where 
they  will  find  more  extended  accounts  of  the  results  embodied 
in  the  above  and  other  papers  than  were  possible  in  a  mono- 
graph of  the  dimensions  of  the  present  work  : — 

"  The  Chemistry  and  Physics  of  Dyeing,"     Dreaper. 

"  Die  Theorie  des  Farbeprozesses,"          -      L.  Pelet-Jolivet. 


INDEX 


ABSOKPTION  of  dyes,  regulation 

of  rate  of,  18 

Absorption  stage  of  dyeing,  69 
Adhesion  of  dyes  and  fibres,  33, 

71 

Adjective  dyes,  34 
Adsorption  theory  of  dyeing,  56 
Amphoteric  substances,  7,  35 
Assistants,  18 
Auxochrome  groups,  13 

BASTOSE,  5 

CELLULOSE,  4,  51 
Chemical  theory  of  dyeing,  34 
arguments  in  support  of, 

35,  45 

objections  to,  40,  43 
Chromogen,  12 
Chromophore,  12 
Colloidal  theory  of  dyeing,  62 
Colloids,    precipitation  of,   by 

dyes,  63 
protective    influence    of,   on 

benzopurpurin,  65 
Colours,  ice,  17 
mineral,  16 
sulphur,  14 

Condition  of  dyes  in  solution,  20 
Cotton,  4,  35 
action  of  acids  on,  4 
action  of  alkalis  on,  5 
action  of  hypochlorites  on,  5 


DIALYSING  power,  influence  of 
molecular  complexity  on, 
22 

Dialysis  of  dye  solutions,  21 
anomalous  behaviour  of  dye- 
stuffs  on,  27 
Distribution  of  dyes  between 

fibre  and  dyebath,  54,  59 
Dyeing,  absorption  stage  of,  69 
definition  of,  1 
development  of,  1 
divided  nature  of,  process,  68 
examples  of,  occurring  in  two 

stages,  71 

fixation  stage  of,  70 
influence  of  state  of  fibre  on 

rate  of,  57 
non-reversibility  of,  process, 

52,'  72 

of  inorganic  substances,  58 
theories  of,  31,  34,  49,  56,  60, 

62,  71 

uses  of  assistants  in,  18 
Dye-bases,    two    modifications 

of,  42 

Dyes,  acid,  14,  35,  46,  60,  62 
basic,  13,  35,  45,  60,  62 
developed,  16,  72 
direct  cotton,  14,  59,  62 
distribution      of,      between 
fibre  and  dyebath,  54,  59 
mineral,  16 
molecular  complexity  of,  22,29 


80 


INDEX 


Dyes,  mordant,  15,  51,  72 
sulphur,  14 
vat,  15,  72 

ELECTRICAL  attraction  between 
fibres  and  dyes,  60,  63,  69 

FIBRES,  3 

absorption  of    dyes    by    in- 
organic substances    and 
by,  58 
nature  of  the  reactive  groups 

in,  47 

Fibroin,  8,  51 
Fixation  stage  of  dyeing,  70 

GLANZSTOFF,  10 

HYDROLYSIS,  membrane,  28 
of  dyes  in  solution,  20,  23,  41 
products  of,  of  wool,  7,  36 

Hypochlorites,    action    of,    on 
cotton,  5 

IONISATION  of  dyes  in  solution, 
20,  25,  29 

JUTE,  3,  5 
KERATIN,  6 

LANUGINIC  acid,  7,  36 
Linen,  3,  4 

MECHANICAL  theory  of  dyeing, 
31,34 

objections  to,  33 
Membrane  hydrolysis,  28 
Mercerised  cotton,  5 
Molecular  complexity,innuence 

of,  on  dialysing  power,  22 


Mordant  dyes,  15,  34 
Mordants,  14,  15 

OXYCELLULOSE,  5,  40 

KATE  of  dyeing,  18,  57,  61 
Reactive  groups  in  fibres,  47 

SERICIN,  8 
Silk,  8 

action  of  acids  on,  9 
action  of  alkalis  on,  9 
artificial,  9 
viscose,  10 
Solution,  condition  of  dyes  in, 

20 

theory  of  dyeing,  49 
arguments  in  favour  of,  50 
objections  to,  52 
results  favourable  to,  59 
another,  59 
Surface  action  of  fibres,  56,  69 

TEMPERATURE,  effect  of,  on 
condition  on  dyes  in  solu- 
tion, 27,  66 

Theories  of  dyeing,  31,  34,  49, 
56,  60,  62,  71 

UNEXHAUSTED  dyebath,  prob- 
lem of  the,  44,  51,  72 

Union  between  fibre  and  dye, 
nature  of,  35,  48 

VISCOSE  silk,  10 

WOOL,  6 

action  of  acids  on,  8 
action  of  alkalis  on,  8 


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